[admin] Create users and groups from witihin wdb+ admin pages

[dariok]

eXist’s dashboard has a user management but that is currently (5.2) rather buggy. Also, it requires the user to be dba which may not be the case.
If we implement our own user management, we can limit that to the wdbadmin group (with setuid active) and need not use the dashboard. In the end, the dashboard could largely be disabled which is a srcurity plus.

Our own user managment needs

• create user
• create group
• assign user to group
• assign project to group or user

This should be done in a RESTful way as it will be used by #382 among others.