UX: prosemirror rich editor nodes cleanup / slightly better UX (#32361)

We don't want "ghost" `content` within mention/hashtag, as they're
already rendering their non-editable content on `toDOM`.

`atom: true` is not necessary for content-less nodes

Re-use existing heading node spec instead of re-creating.

A UX improvement from these changes is a better Cmd-Left/Home navigation
when these nodes begin a paragraph and the caret is after them.

plugins/discourse-local-dates/assets/javascripts/lib/rich-editor-extension.js

@@ -5,7 +5,6 @@ const extension = {
     local_date: {
       attrs: { date: {}, time: {}, timezone: { default: null } },
       group: "inline",
-      atom: true,
       inline: true,
       parseDOM: [
         {
@@ -42,7 +41,6 @@ const extension = {
         timezone: { default: null },
       },
       group: "inline",
-      atom: true,
       inline: true,
       parseDOM: [
         {