Add a default secret for the ushahidiui client, refs T258

Summary:
- remove broken migration that tried to add the client
- add working migration the UshahidiUI module that uses config
- update Base.yml to match

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

Reviewers: rjmackay, dkobia

Subscribers: eyedol, brianherbert

Maniphest Tasks: T258

Differential Revision: https://phabricator.ushahidi.com/D130