Stop using email address as proxy for user object

[sm6xmm]

After having "solved" the issue of email addresses should be compared case insensitive several times (there are still corner cases where the case of the email address matters), it has now also popped up in the form of "users can edit their email address". This has the effect of (possibly amongst others) of making tickets appear unassigned, because they use the email address as a proxy for the user. (The reason for this is that it used to be possible to assign tickets to people who were not yet in the system.)

So we need to find all places where email addresses are used to look up the user object, and where possible replace it with a direct reference to the user object.

This is probably a rather large task, and may need to be split up.