Use Endpoint for every API call in tags, refs T696

Summary:
- cleanup on Endpoint class
- add REST base controller
- add use cases for "get index" and "get collection"
- add endpoints for all tags API calls
- inject `allowed_methods` using formatter
- handle formatter for collections
- move endpoint service registrations to `src/Init.php`

Test Plan:
```
bin/phpspec run -f dot
bin/behat -f progress -c application/tests/behat.yml application/tests/features/api.tags.feature
bin/behat -f progress -c application/tests/behat.yml application/tests/features/api.acl.feature
```

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: spaudanjo, eyedol, aMoniker, rjmackay

Maniphest Tasks: T696

Differential Revision: https://phabricator.ushahidi.com/D327