Clean up creation of the ArchivesSpace software agent and system
users.

Get rid of hard-coded IDs and lists of usernames where possible.
Backend changes for allowing users to be marked as admins
Add a new option to the user create/edit form to mark a user as an admin user.
Tweak from code review
Make sure people can't inadvertently remove all admins
Remove hard-coded URI for global repository
Merge pull request #505 from hudmol/additional-admins

Allow users to be made into admins.