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`)
