FEATURE: Show error message to blocked users

[tillprochaska]

Is your feature request related to a problem? Please describe.
Aleph ships with an option to block individual users. Right now, when a blocked user tries to sign in, they get a generic "Invalid credentials" or "Unauthorized" (depending on auth flow) message. Depending on a reason for which a user has been blocked, this can be confusing to users.

Describe the solution you'd like
We want to be able to show a helpful error message when a blocked user tries to sign in. The message should be configurable. There should be a way to configure a link to an external webpage with more information.

Describe alternatives you've considered
-/-

Additional context

• If a users is blocked, the is_actor property evaluates to False. The is_actor property is used during the OAuth flow and the password flow.
• Right now, both flows treat blocked users basically in the same way as a nonexistent user and a generic "Invalid credentials" message is displayed.

•  Check test coverage for auth flows
•  Redirect blocked roles during OAuth flow (code)
• Redirect blocked roles during password flow (code)
• Decide whether to redirect blocked users to a separate (new) UI screen or to a configurable page (e.g. the "FAQ" page).
• Ensure that API key authentication remains unaffected