Adds a log in/out button

Note that solving and editing form still need to handle 401 response.

The gae implementation partially implements it:

- Solving should required a student in user (gae implementation currently require a logged in user).
- Editting problem should required an admin or staff member (gae implementation currently require just a logged in user).
Merge pull request #55 from dinoboff/tweak-api

Adds a log in/out button