notifications: Add stream email notifications.

[shubhamdhama]

This completes #7788.
@timabbott FYI.
I've fixed some minor and major merge conflicts so it's better to see a diff between original and these commits.

[timabbott]

@shubhamdhama can you rebase this? It's like 1000 commits behind master :)

[shubhamdhama]

@timabbott Resolved conflicts, though I'm not completely familiar with changes to how to test whether it does its job properly or not?

[showell]

FYI @shubhamdhama and I chatted about how to test this, and he's going to work on that today and ask questions on chat if any come up.

[timabbott]

@shubhamdhama do you have an update on the status of the work you and Steve discussed? Maybe something for us to chat about in person at the retreat tomorrow.

[showell]

I'm guessing this just kinda got lost in the shuffle. My advice is for Tim to just look over the shoulder of @shubhamdhama as he manually tests this, just to make sure all the new email niceness we have in dev works as advertised and to give any handy tips for manual testing in general.

[shubhamdhama]

Sure we can discuss this tommorow!! BTW here is the link to the screenshots of the mails: https://chat.zulip.org/#narrow/stream/6-frontend/subject/testing.20emails/near/609440

[timabbott]

@shubhamdhama and I did a rebase/split of this into a longer commit series, fixing a few bugs and several more things that were confusing about this PR. I then merged this as the commit series ending with de2ec8a.

Huge thanks to @sarahcstringer and @shubhamdhama for all their work on this!!!

Here are a couple follow-up projects we should probably do:

• Fix messages[0].header.plain to be more readable; I think we can use the stream_name value that was passed alongside trigger.
• Can we improve how test_end_to_end_missedmessage_hook does the database setup to use the do_foo function rather than hacking up the object?
• @rishig I bet there's some documentation updates we need to make following this being merged; can you make sure those happen?