Complete architecture for Form endpoint, refs T848

Summary:
- convert controller to REST
- add DI mappings and supporting classes
- reuse read/write when possible

Test Plan:
```
bin/behat -c application/tests/behat.yml -f progress application/tests/features/api.forms.feature
```

Reviewers: vladimir, sheesh, aMoniker

Reviewed By: aMoniker

Subscribers: spaudanjo, rjmackay, srutto

Maniphest Tasks: T848

Differential Revision: https://phabricator.ushahidi.com/D475