Outgoing message improvements

[rowanseymour]

• Splits the generic message sending endpoint into three more logical endpoints for case replies, bulk replies and forwards
• Records contacts and URNs on the outgoing message
• Fixes a bug on the forwarding modal dialog

[rowanseymour]

Previously the pattern for creating something that exists in both RapidPro and CasePro, was

1. Use an API call to create it in RapidPro
2. Get the identifier used by RapidPro
3. Create locally with that identifier

But for things that aren't being pushed to the backend - this pattern makes more sense:

1. Create locally
2. Optionally push to backend and update with returned identifier

Another advantage of this is that the local outgoing message is still created even if the RapidPro API call fails.

[coveralls]

Coverage decreased (-0.1%) to 94.638% when pulling 1017c42 on message_sending into 1ef04e6 on master.

[nicpottier]

It's so beautiful!

[coveralls]

Coverage increased (+1.1%) to 95.858% when pulling c6c6a53 on message_sending into 1ef04e6 on master.