Editor: Remove global state placeholder actions

[aduth]

This pull request seeks to effectively revert #3025 and #3202, removing placeholder actions from being called in the post editor, for the following reasons:

• The intended direction for editing posts via global Redux state has changed dramatically since the original placeholders were added:
  • Action creators now exist in state/posts/actions, not state/ui/editor/actions (source)
  • When editing a post, one must specify the site ID and optional post ID to the action creator (source)
  • We may not want specialized action creators per field
  • We were premature in making assumptions about how Redux state would be structured, e.g. we can refactor how we identify whether editing a new post in place of our previous approach in explicitly dispatching actions for editing sessions of new or existing posts
• Dispatching placeholder actions has a negative effect on performance
  • Dispatching to the global store, even if there's no effective change in state, causes all mounted connect components to re-run their state selector logic
  • Subtle issues like getContent (a very slow function) being called multiple times
• Inconsistently applied in newer refactorings (example)
• Causes confusion to developers who may not understand the intent of the duplicated actions
• Progress has stalled on fleshing out behavior of placeholder action creators

Testing instructions:

Ensure Mocha tests pass:

npm run test-client

Verify that there are no regressions in all affected files, which is effectively all editor behavior which has an effect on the values saved for a post.

Ensure no remaining references to state/ui/editor/post exist.

[gwwar]

This looks good so far @aduth. Ping me when you rebase and I can test again.

[aduth]

Thanks @gwwar ! Rebased (single semi-colon caused it to conflict 😆 ).

Also added note to testing instructions about verifying what's been removed.

[gwwar]

🚢