Add DBUserCheck to create a Roles object from the DB (See #11813)

By default the strings "user", "guest", "root", and "system" are
used to look up the IDs. By setting omero.roles.user.guest etc.
in config.xml, other strings can be used, but this is NOT suggested.
Add dbUserCheck to background processes
Add missing file
Clarifications thanks to @bpindelski
Merge pull request #1894 from joshmoore/11813-Roles

Add DBUserCheck to create a Roles object from the DB (See #11813)