auth: Split off vote data from the user

Now that the application is going to have multiple pages, vote data makes no sense
to keep with the user.
The user struct will be used everywhere to check for authentication, which is not
the case for previous votes.

Create a new struct and function in src/vote.rs to retrieve existing votes and
use them in places where user.votes was used previously.
Remove vote-related code from src/auth.rs and the week number dependence that it
required.

README.md

@@ -19,6 +19,7 @@ A list of things that could be implemented/added to the application, some of the
  - [x] Use fairings for the different elements
  - [ ] Use guards for User calls ?
  - [ ] Use SQLite Row ID for User IDs rather than regular IDs, for randomness ?
+ - [x] Split user from vote data
 
 # Dependencies