Add blacklist for groups and ignore blacklisted roles in Discord service

Per our discussion this change aims to enable the Discord service to work with other Discord bots that use their own roles:

• New feature Reserved group names
  • New model "reserved group names"
  • Admin site: New reserved group names entries can not match with existing groups
  • Admin site: Groups can no longer be saved when their names are reserved
  • When groups are saved programmatically their names are automatically adjusted in case they are reserved (e.g. test -> test_1)
• Enhanced Discord service
  • Can now persist roles when updating groups for a user
  • Roles to persist are defined through reserved group names. Matching of names is case insensitive.
• Includes partial refactoring of groupmanagement and discord service
• Includes tests for all new and modified code
• Documentation
  • Previous pages for "groups" and "group management" have been consolidated on one page
  • Description for restricted group names has been added
  • Updated Discord service about this new feature