FEATURE: explain why invites are disabled to staff users (PR #10810)

This commit improves error messages (for staff) when invite system is disabled because:

  • SSO is enabled
  • enable_local_logins setting is disabled

Screenshot from 2020-10-02 23-04-33


      local_logins_disabled: "Invites are disabled because the 'enable local logins' setting is disabled."

Suggestion: Use a helper method for when there is an error such as render_invite_error(message) - this code is the same as used above.