Test message capability

[mfb]

We would like to have the ability to create draft messages, send it to a saved set of staff / testing device IDs, then have one click to send the message to full list of subscribers.

Also should be possible to edit the message if necessary, e.g. by clicking back.

[maximillianh]

When sending a draft message, we want to be able to select from two test groups: one for sysadmins, and one for activists.

There's no need to prepend [Draft] to the test messages -- they should appear exactly as subscribers would receive them.