Cleans up obsolete code related to posting.

Removes orphaned / dead code.
Removes iOS6 / iOS7 branching. Favors iOS7 only.
Removes unused IBOutlet and IBAction declarations.
PostMediaViewController now inherits from UITableViewController.
Deleted obsolete xibs for PostMediaViewController
Removes references to EditPostViewController in PostMediaViewController and PostSettingsViewController.
Misc. other clean up.
Refs #686 

There's probably more that could be done but this is a good start and will make later improvements easier to manage.