Add support for other username-password auth providers
This adds support for
- other username-password auth providers, e.g. LDAP, and
- mutliple enabled username-password auth providers
If multiple UPAPs are enabled, instead of a single 'Login' button multiple buttons are displayed using the title from the API configuration: e.g. 'Login with LDAP' ('LDAP' is the title).
Implements: #678 (closed)