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

- [x] Filter title by keyword
- [x] Active competition filter
- [x] Ended competition filter
- [x] Medical image viewer