[WIP] Add ssh keys to settings API

This PR adds settings API for ssh_keys. `SettingsEndpoint` is written in a way that we can use any settings collection to automatically create needed actions.

I also added active model serializers here which can be used interchangeably with regular API payload generators.

After defining a collection in `travis-core`, adding it to API should be as easy as `SettingsEndpoint.subclass(:notifications)`