Move post/page creation to PostService
Implement post sync in PostService

This adds a REST implementation for posts sync but also had to refactor
quite a few things.

Some notes:

- PostServiceRemoteXMLRPC uses `wp.getPosts` instead of `metaWeblog.getRecentPosts`.
- There are a few FIXMES that should go once the relevant fixes for
  wp.com REST API go live.
- Post format selection fails since I removed the post format sync.
  Post formats are included in rest options, but need to be parsed.
  They need a separate call on XML-RPC.
- I removed blog sync from the PostsViewController pull-to-refresh.
  Featured images rely on Media objects to be present and it only works
  because it syncs the media library on login. We could just store
  {ID,URL} since that's all we need for featured images.
- Categories are now created from the posts response, but we need to
  make sure they refresh when the category selector is displayed.
- In general, more testing is needed regarding everything that was
  synced on post refresh before. We can't depend on the user refreshing
  posts to load formats or categories.
Updated comment for post thumbnail check
Added infinite scroll for posts using REST
Create/update posts using PostService

Adds REST implementation
Merge remote-tracking branch 'origin/develop' into issue/1901-rest-posts

Conflicts:
	WordPress/Classes/Services/CategoryService.m
	WordPress/Classes/ViewRelated/Post/WPLegacyEditPostViewController.m
	WordPress/WordPress.xcodeproj/project.pbxproj
Fix method name from bad merge
Use PostService to delete posts
Add Page support to PostService
Look at all that code that won't be run again
A temporary comment view controller created and basics of updating to the new design has been done. Once the temporary version is ready, I will switch it over and remove the old one
Fix seconds on JSON date parser
Removed API workaround
Merge remote-tracking branch 'origin/develop' into issues/2317-update-comments-page
like, approve, trash actions implemented for new comments page
Merge remote-tracking branch 'origin/develop' into issues/2317-update-comments-page
Display more actions, spam and edit comment implemented for new comments page
Merge branch 'develop' into issue/1901-rest-posts
Merge remote-tracking branch 'origin/develop' into issues/2317-update-comments-page

Conflicts:
	WordPress/WordPress.xcodeproj/project.pbxproj
Podfile.lock updated