Revise email docs and update email configuration

[czlee]

Will close #1751, once it's ready.

Notable changes:

• Replaced config vars SENDGRID_USERNAME and SENDGRID_PASSWORD with just SENDGRID_API_KEY—this is more aligned with how personal SendGrid accounts should be used, and since we're dropping automatic provisioning of the SendGrid add-on, there's no need for us to follow their convention. Plan is to check for the old setting and for it still to function, but to show a warning on the tournament overview page.

More to come—may add more notes on other options to docs. Thoughts welcome while still being drafted.

[czlee]

Quick note while I remember:

• This PR should add a deprecation notice that is displayed to users who are still using SENDGRID_USERNAME and SENDGRID_PASSWORD, since support for that will eventually be removed, and it is indicative of users who are using the Heroku add-on. (This is noted above, but I'm sure I'll forget next time I look at this if I don't add a to-do note.)

[czlee]

Random thought:

• I think we can control the from email address from source code (as opposed to from DEFAULT_FROM_EMAIL), so it's probably not a bad idea to make this a proper Tabbycat setting and run some more intelligent checks/error messages when sending fails due to Sender Identity Verification or something like that.

[tienne-B]

I think we can control the from email address from source code

Yes we can.

[czlee]

Current status:

• This removes the auto-provision SendGrid, and changes the expected config var to SENDGRID_API_KEY, as indicated earlier.
• It shows a deprecation message if it detects that SENDGRID_USERNAME is in use.
• Docs are updated accordingly.
• It makes an effort to catch the SendGrid Sender Identity error.

Caveats:

• I don't know whether the Sender Identity error catch actually works, and I'm not sure how to test it because short of creating a new account I'm not sure how to generate the error. I think it's probably an SMTPResponseException but I couldn't say for sure. I've added logger.warning(..., exc_info=True) calls throughout the code catching test email errors to try to give us better visibility.
• I haven't created the "from email" preference, because I realized as I was doing it that I wasn't sure whether to make it a site-wide preference or a tournament preference. My first instinct was tournament preference, but that would break the test email facility, which is a site-wide page. I'm inclined to leave this for a future PR/commit unless other devs think the answer here is obvious.