Add custom activation email message

[jdavcs]

Fixes #10607.

Is this sufficient?

[hexylena]

@dickgroenenberg is this fine for your needs?

[dickgroenenberg]

Yes, very convenient. Thanks for adding this feature Sergey and Helena.

[hexylena]

@ic4f should there be a test for this function? If there was I'd be happy to hit merge.

[jdavcs]

@ic4f should there be a test for this function? If there was I'd be happy to hit merge.

@hexylena no, it doesn't have a test, and you're right - it should have a simple unit test (same for the enclosing method, send_activation_email, which needs several). I've added it to my testing todo list; if you don't mind I'll take care of this later. And you are completely right - little trivial changes like this one are good triggers for refactoring, adding tests, and cleaning up the code base.

[hexylena]

@ic4f sure, no worries, no time pressure for sure. I was just remembering marius suggesting a while back that reviewers should ask for more tests and thought it might not be a huge burden here. :)

[jmchilton]

Opps - sorry I really didn't even see that comment. I was just going through and clicking merge on stuff that had stalled out and looked fine to me. Happy to also merge a test 😆

[jdavcs]

I think I also brought up the same point at that discussion - asking for more tests on such PRs. Now, this is embarrassing :) I'll add the test of course! Thanks, @hexylena , @jmchilton