[WIP] Admin tests

This pull request adds more unit tests to Wagtailadmin.
 
TODO:
 - [x] Split up wagtailadmin tests into separate files

Write tests for the following:
 - [x] Account management
 - [ ] Page preview views
 - [ ] Moderation views
 - [ ] Page chooser
 - [ ] Edit handlers
 - [ ] Template tags
 - [ ] Userbar