Add layers API and show layers on the map

Summary:
- Add MapView.addOverlay API for show extra layers on the map
- Add a layer entity and repo
- Add layers table to db
- Add layer api endpoint
- Load and display layers from the API
- Add layer authorizer
- Skip the global ACL check for layers API
- Remove koauth.php - confusing

Test Plan:
- Run migrations `bin/phinx migrate -c application/phinx.php`
- Run tests `bin/behat -f progress -c application/tests/behat.yml application/tests/features/api.layers.feature`
- Go to homepage and view layers (have to run tests first for data)

Reviewers: bodacea, eyedol, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T53

Differential Revision: https://phabricator.ushahidi.com/D298