Split AddEntryActivity

This pull request splits up the ```AddEntryActivity``` into two separate classes: ```AddEntryActivty``` and ```EditEntryActivity```. They are both a subclass of the new ```AbstractEntryEditorActivity```, which holds the shared code between the two.

Essentially a refactor of ```AddEntryActivity```. Changes no functionality. Splits up what used to be one super-entry class into two simpler, easier-to-understand and read classes.