Recreate tag create use case and tests, refs T238

Summary:
- move away from Collection style
- implement tag create in api endpoint
- parsers and validators work with plains arrays
- final use case action is to create an entity
- exceptions are thrown in use case
- better spec tests

Test Plan:
```
bin/phpspec run spec/Ushahidi/Usecase/Tag/CreateSpec.php
bin/phpspec run spec/Ushahidi/Entity/TagSpec.php
```

Login/Register tests are broken by this diff and will be addressed separately.

Reviewers: dkobia, rjmackay

Reviewed By: dkobia, rjmackay

Subscribers: eyedol

Maniphest Tasks: T238

Differential Revision: https://phabricator.ushahidi.com/D112