[#856] Changes based on Gabriel's code review

Several changes have been made;
- ProjectUpdateExtraResource has been deleted.
- AuthBackend is now based on the default Django backend.
- Avatar help_text shows 500 kb instead of 3.5 mb.
- Request to join organisation process and emails updated.
- Small changes in the User and Employment models.
[#856] Updated URLs and added TastyPie API
Merge pull request #888 from akvo/feature/856_v3_code_review

[#856] v3 code review