Add broadcasts

[stympy]

We have some code that we use at Honeybadger for sending broadcasts with Heya:

https://gist.github.com/stympy/3d76041365a61085ddcf01a3461f65f1

We should bring that into Heya, and also have a web UI for sending broadcasts... perhaps a mountable app like sidekiq has. Maybe a basic V1 could just have a text box for taking the markdown content of the email, and then some way to specify a list of recipients. An ActiveRecord query to run via eval? A list of user ids? Suggestions are welcome. :)

[joshuap]

I kind of like the list of user ids option as a temporary UI. We'll want some kind of query builder eventually (with the ability to save named queries/segments). 🤔

I had considered adding "saved segments" which would let you define segments using a similar method to campaign segments, but the problem with that is if you allow arbitrary Ruby segments, you need to maintain the members in a separate table (e.g. users would need to be added/removed automatically via callbacks)— assuming we wouldn't want to iterate over every user in the database when displaying the segment or sending a broadcast. ;)

[johnknapp]

Ahoy @stympy and @joshuap, super excited and impressed with heya! We're looking at using it to roll our own emailer and this gist looks handy! We're just breaking ground so I suspect you'll be hearing more from us in future issues and PRs. Cheers, JK