Search + filtering for competitions

Resolves issue #22 

## Functional review
- [ ] Go to competition list ("Competitions" in nav bar, left of "My Codalab")
- [ ] Test filter by keyword
- [ ] Test active filter
- [ ] Test ended filter
- [ ] Test medical image viewer filter


## Tasks

- [ ] Filter title by keyword
- [ ] Active competition filter
- [ ] Ended competition filter
- [ ] Medical image viewer