Fix bugs on develop branch

- Fix error after deleting validation condition and text. http://manage.dimagi.com/default.asp?110062
- http://manage.dimagi.com/default.asp?110331
  - Allow Label questions to be changed to other question types.
  - Show type changer button on Choice and Hidden Value questions. The list of "Change to..." options is empty except for a label that says "Cannot Change Question Type". The reasons for this are:
    - The type changer button should always be visible because it is the primary way to tell what type of question is being edited.
    - Hiding the type changer button effectively puts the Copy button in its place, which is inconsistent and bad for locational memory.
    - We are not disabling the button because disabled buttons are sometimes hard to read and cause frustration.