[#933] First version of 'Add an update' page

- Added a page for adding updates
- Only accessible for those allowed to add an update
- If on a project (update) page, user is directed to the update page directly
- Otherwise, the user goes to the 'My Projects' page in MyRSR
- Added an 'Add an update' link to every project in 'My Projects', if the user is allowed to post updates for that project

To do:
- Add location of update
- Cleanup code
[#933] Added location to update form
[#933] Update edit mode and small lay-out changes to update-main
[#933] Code cleanup
[#933] Merged 'feature/rsr_v3' into branch
[#933] Cleanup more code
[#933] Merged 'feature/rsr_v3' into branch
Merge pull request #940 from akvo/933_add_update

[#933] Add an update page