When LDAP is enabled but password login is disabled, login doesn't work

[noahmoss]

Describe the bug
If you set up and enable LDAP, but toggle off the "Enable Password Authentication" setting, the login screen will say "Sign in to Metabase" not present any authentication options.

To Reproduce
Steps to reproduce the behavior:

1. Go to Admin -> Settings -> Authentication
2. Set up and enable LDAP. The easiest way I've found for this is to use these credentials for a free online testing server, but you can also run one locally.
3. Scroll down to "Enable Password Authentication" and disable it
4. Log out
5. The login screen will not present any authentication options

Expected behavior
I guess we should still show the login form but only attempt to hit the LDAP server when you submit it, not attempt normal email/password login as well

Screenshots

Information about your Metabase Installation:
Current master

Severity
Pretty serious, it's an easy way to get locked out of an instance. Can be recovered from by setting MB_ENABLE_PASSWORD_LOGIN=true.