priority index migration

Summary: adds/removes an index on `priority` for `form_attributes`

Test Plan:
`bin/phinx migrate -c application/phinx.php` to add
`bin/phinx rollback -c application/phinx.php` to remove

check that the index is there by running `show index from form_attributes;`

run `bin/tests` for extra good luck

Reviewers: vladimir, shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay

Maniphest Tasks: T1092

Differential Revision: https://phabricator.ushahidi.com/D493