Remove deprecated viewDidUnload

Places clean up tasks in appropriate dealloc or didReceiveMemoryWarning.

Removes WPWebAppViewController as it doesn't seem to be used.
Drop .png from bundle images
Use weak references to views/delegates

Continued from removing viewDidUnloads:
  - try to use weak references when possible to avoid needing to assign
    to nil on dealloc/didReceiveMemoryWarning
  - For views and delegates