Add ability to attach a role/permissions to a tag

Summary: Created a migration to add a "role" column in the tag table and the UI to handle this - add/edit a tag

Test Plan:
On the workspace menu select 'Manage Tags', on one of the tags select 'edit' and try change the role.
bin/behat --config application/tests/behat.yml application/tests/features/api.tags.feature --format progress

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: brianherbert, eyedol, srutto

Maniphest Tasks: T182

Differential Revision: https://phabricator.ushahidi.com/D233