Avoid sending user emails if @ mentioning a staged user
Some cases, unknowingly mentioning a staged user would invite
them into topics, sending them an email about it.
When user is watching category or tag (watching or watching first post) notifications are moved to other tab.
To achieve that and distinguish between post create to directly watched topics and indirectly watched topics, new notification type called `watching_category_or_tag` was introduced.
This fix ensures that if a staged user is linked to or quoted they won't
be emailed about it.
A staged user could email into a category, and another user could quote
them inside of a completely different category and we don't want a
staged user to receive an email for this.
Bug report:
https://meta.discourse.org/t/-/145202/9
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Migrates email user options to a new data structure, where `email_always`, `email_direct` and `email_private_messages` are replace by
* `email_messages_level`, with options: `always`, `only_when_away` and `never` (defaults to `always`)
* `email_level`, with options: `always`, `only_when_away` and `never` (defaults to `only_when_away`)
