Add default "admin" user with migration, refs T613

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

Reviewers: rjmackay, bodacea

Subscribers: dkobia, eyedol, srutto

Maniphest Tasks: T613

Differential Revision: https://phabricator.ushahidi.com/D307