Replace regular login with MMT OAuth

As a user I want to log in using an SSO Service, that is also in use for other FH services, so that I don't have to retype my credentials as often.

Acceptance criteria:

• Login page must redirect to the OAuth service
• The login must only be possible when the user is already in the database
• The user must be able to log out of the Portfolio page (this does not have to include a log-out of the SSO service)
• Optional: you can still reach the old login page but SSO should be the default behavior

Chosen OAuth service: https://auth.projects.multimediatechnology.at/

Implementation: Use Devise OmniAuth to implement the service.

To Do's (WIP):

• Dive into Rails Devise and OmniAuth
• Get user records to test
• Add MMT SSO as OmniAuth login provider
• Integrate provider with Portfolio
• Adjust login flow

Expected impact on production:

• Service needs to be added to OAuth Server
• Application UID and Secret need to be added to the ENVs

Responsible: @fhs50537 @fhs44311