Add an interface for task views

This should be implemented by all task views, and eventually TaskPresenter
will use this instead of using hardcoded TaskView objects directly.

Change-Id: I02ce0e4dd07b0c016955b4d270f155ed60b94b7a
Add an interface for activity views

This should be implemented by any view that displays an activity, such as the
view to input a comment and the view to render an individual activity.

Change-Id: I49eee6caa3501f90edad0e96c82d88dda554697d
Merge "Add an interface for task views" into refactor/interfaces