Add updateConfig helper, use in SettingsView, fixes T317

Test Plan:
- Login
- Go to settings page
- Save site settings
- It works!

Reviewers: eyedol, lkamau, rjmackay

Reviewed By: rjmackay

Subscribers: dkobia, sebm90

Maniphest Tasks: T317

Differential Revision: https://phabricator.ushahidi.com/D139