Add Experimenter initiated Alerting

[jimbojw]

Fixes #757

Description

Adds a Message button to Participant entries in the cohort dashboard. When clicked, it opens the Experimenter alerting panel where they can send an outgoing message to a participant.

Verification

Screenshots showing the Messagebuttons for Participants in the cohort dashboard:

Screenshot showing the new outgoing message dialog:

Screenshot showing outgoing message received by participant:

[cjqian]

Some high level feedback:

1. After I click "Message participant", I'd like for that participant's information to already be loaded in the dropdown (so I don't have to manually find them again) with cursor ready to go in the typing box. (image 1)

2. What does it mean to message an agent participant (image 2)? Should we deactivate for agent participants and put in documentation that you can't message agent participants, or should we add additional logic to do this? Leaning towards the former.