Don't cache CommentService in CommentViewController

Since services are designed to be disposable, we shouldn't cache a CommentService instance in CommentViewController and an instance should be created when it's needed.
Merge pull request #2968 from wordpress-mobile/issues/2941-dont-cache-commentservice

Don't cache CommentService in CommentViewController
Fixes comment page header setup crash

authorAvatarURL removed from Post & ReaderPost and added to BasePost instead (in Core Data model). Unit test added to test the migration.
Merge pull request #2969 from wordpress-mobile/issues/2945-comment-page-header-crash

Fixes comment page header setup crash
Merge remote-tracking branch 'origin/develop' into issues/2755-tabbar-enhancement

Conflicts:
	WordPress/WordPress.xcodeproj/project.pbxproj
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