Add user detail view and actions

[ralf401]

• User Details
• How to create a new user as agent
• How to change an existing user
• User history
• Create ticket from user details
• Delete user from user details (admin permission required)
• Maybe create an FAQ how to deal with unknown customers when receiving a call
• Maybe add that a ticket can be created via link which opens the edit form (optionally even with pre-populated customer)
• TBD