Refs #17748 - Filter out who to send the emails to

[adamruzicka]

What are the changes introduced in this pull request?

Previously the emails were sent out to all the users which opt-in for it, even if those users were disabled or didn't have mail notification checkbox enabled.

What are the testing steps for this pull request?

1. Set up a user, opt in for repo sync failure email notifications, disable the user
2. Run a repo sync that will fail
3. The user from 1 shouldn't get an email

[pnovotny]

LGTM, tested with the Packit builds.
Disabled user or user with disabled email doesn't receive the email notification about repo sync failure.

[ofedoren]

LGTM as well, not sure whom I can ping here though :/ maybe @jeremylenz could take a look? :)

[jeremylenz]

Acking based on @pnovotny and @ofedoren

Thanks all!