Validate data sources config params, fixes T326

Summary:
 - Add validation rules for smssync secret field
 - Support executing closures on the rules field
 - Support validators in the form scheme
 - Call `form.validate()` to validate form fields

Test Plan:
 - Go to messages/settings
 - Click configure on SMSSync data provder
 - Submit empty value for the secret
 - See required error message displayed around the secret field
 - Submit a value for the secret key and see pass

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: srutto, lkamau

Maniphest Tasks: T326

Differential Revision: https://phabricator.ushahidi.com/D286