Improve title parsing

The subtitle has been removed because it's difficult to find and error
prone to guess at. It is somewhat accessible from the post's
previewContent field in GraphQL but that can be truncated.

spec/components/page_content_spec.cr

@@ -6,7 +6,6 @@ describe PageContent do
   it "renders a single parent/child node structure" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -24,7 +23,6 @@ describe PageContent do
   it "renders multiple childrens" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -53,7 +51,6 @@ describe PageContent do
   it "renders an anchor" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -69,7 +66,6 @@ describe PageContent do
   it "renders a blockquote" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -87,7 +83,6 @@ describe PageContent do
   it "renders code" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -105,7 +100,6 @@ describe PageContent do
   it "renders empasis" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -127,7 +121,6 @@ describe PageContent do
     children = [Text.new("A caption")] of Child
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -155,7 +148,6 @@ describe PageContent do
   it "renders a GitHub Gist" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -173,7 +165,6 @@ describe PageContent do
   it "renders an H3" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -191,7 +182,6 @@ describe PageContent do
   it "renders an H4" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -209,7 +199,6 @@ describe PageContent do
   it "renders an image" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -231,7 +220,6 @@ describe PageContent do
   it "renders embedded content" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -256,7 +244,6 @@ describe PageContent do
   it "renders an embedded link container" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -280,7 +267,6 @@ describe PageContent do
   it "renders an mixtape embed container" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -311,7 +297,6 @@ describe PageContent do
   it "renders an ordered list" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -330,7 +315,6 @@ describe PageContent do
   it "renders an preformatted text" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -348,7 +332,6 @@ describe PageContent do
   it "renders an preformatted text" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -366,7 +349,6 @@ describe PageContent do
   it "renders strong text" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -384,7 +366,6 @@ describe PageContent do
   it "renders an unordered list" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [
@@ -403,7 +384,6 @@ describe PageContent do
   it "renders a user anchor" do
     page = Page.new(
       title: "Title",
-      subtitle: nil,
       author: user_anchor_factory,
       created_at: Time.local,
       nodes: [