Add contributors API endpoint

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Collect contributors info via API
Move API contributors logic to repository model
Use Contributor class instead of hash
Add contributors API docs

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'contributors-api' into 'master'

Contributors api

Fixes #1266

See merge request !934