Improve SMSSync workflow, fixes T495

Summary:
 - Throw HTTP 403 exception when there is a missing secret key.
 - Throw HTTP 403 exception when there is a missing from value.
 - Process outgoing messages when SMSSync makes a GET request.
 - Add GET request as a supported HTTP method.
 - Add test scenario for testing missing from value.

Test Plan:
- bin/behat --config application/tests/behat.yml application/tests/features/data-provider/smssync.feature
- ./minion DataProvider:outgoing

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: shadowhand, lkamau, srutto

Projects: #ushahidi_platform

Maniphest Tasks: T495

Differential Revision: https://phabricator.ushahidi.com/D250