Add html select with allowed auth methods

Adds the UI element (when there is only one value, the element is hidden) and refer to it when generating the authorization string.

The requirements of the task #49 (closed) are respected.

Note: This removes a previous behavior where a configured LDAP account creation automatically enforces LDAP login. This is no longer the case as the allowed auth methods are consulted strictly. The previous behavior can be achieved by only allowing LDAP logins.

The implementation strictly uses the values in their spelling (including case) as they are defined by their respective plugin no matter what was spelled in the configuration file, e.g. a list containing only "lDaP" will render a select with the value "LDAP" still because this is the auth method value the LDAP plugin registers.