Option to disable anonymous login

[Schwehn42]

Inspired by #3760, there should be an option to disable anonymous login for those who decide to host scrumlr themselves and want to force users to use third party auth.

This would entail the following:

Backend:

• Add CLI Flag and Config Var for disabling anonymous login (default=false)
• Blocking the relevant routes for anonmous login if the flag is set (see Can't disable anonymous login #3760 (comment))

Frontend:

• Removing or disabing (i.e. gray out) the anonymous login form