[#261] Add "via mobile" choice to update methods

Move rsr.models.UPDATE_METHODS and rsr.models.PHOTO_LOCATION into
ProjectUpdate where they belong.

Add new choice "via mobile" to Project.Update.UPDATE_METHODS.

Fix rsr.forms.ProjectUpdateForm to use the refactoring above correctly.
Merge branch 'develop' of github.com:akvo/akvo-rsr into develop