We generally don't want to delete user accounts, because that will delete all of their associated items as well.  For posterity's sake, we instead choose to disable user accounts that should no longer have access.  However, there are currently only a few places where the is_active flag is considered.

This patch does basically 4 things:

1. Display the is_active flag in the admin user list (/admin/auth/user/).  I'm not thrilled about overriding implementation-specific details, but it works.

2. Filter inactive users from the "All Submitters" list (/users).

3. Filter inactive users from all email notifications, whether they’re addressed specifically or they’re included in a review group.

4. Show a strikethrough on inactive targetted users, to give a visual cue that they won't be included on any further messages.  This way, existing review requests will still show that the inactive user was included.  Inactive users don't show in the auto-complete results, but they can be entered manually.  This is necessary so if you add a new person to a review that already contains an inactive target person, the inactive one won't be lost when it is saved.

There's a slight bug in [4] that the strikethrough is missing when first setting the field.  I don't know how to format the result of the /set/target_people JSON call.

UPDATE (diff revision 2):
- Remove __exact from filters, as it's implied anyway.
- Fix the spacing on '{%' blocks.

I manually verified that the 4 features claimed above seem to work as advertised.