Implement Authorizer for tag visibility

Summary: Implement the authorizer to handle tag visiblity

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

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: brianherbert, srutto, shadowhand

Maniphest Tasks: T182

Differential Revision: https://phabricator.ushahidi.com/D274