* FEATURE: Add `Top Ignored Users` report
## Why?
This is part of the [Ability to ignore a user feature](https://meta.discourse.org/t/ability-to-ignore-a-user/110254/8), and also part of [this PR](https://github.com/discourse/discourse/pull/7144).
We want to send a System Message daily when a specific count threshold for an ignored is reached. To make this system message informative, we want to link to a report for the Top Ignored Users too.
We can only be sure that an email is sent when we get a mailer in
`ActionMailer::Deliveries`. A couple of tests were actually incorrect
because it didn't flow through our email sender where there are more
conditions in determining whether an email is sent or not.
* FEATURE: Account for `ignored_users` when merging two users
## Why?
This is part of the [Ability to ignore a user feature](https://meta.discourse.org/t/ability-to-ignore-a-user/110254/8).
When we merge two users, we need to account for merging their list of `ignored_users` too.
- Notices are visible only by poster and trust level 2+ users.
- Notices are not generated for non-human or staged users.
- Notices are deleted when post is deleted.
It seems that due to jobs being asynchronous and wrapping code in a
DistributedMutex that by the time we run the
`UserAvatar#update_gravatar!` job that the user/user email might be
destroyed.
This patch checks before a call to `user.email_hash` to make sure
the user and primary email exist to prevent the exception. If not
present, the job exits as there's nothing to do because we are
probably running after the user was destroyed for some reason.
