WIP: Option to prevent LDAP sign in

What

Untested prototype to solve https://gitlab.com/gitlab-org/gitlab-ce/issues/20798

Adds setting to prevent using LDAP for sign in.

Why

When using another system such as SAML for authentication it can be desirable to disable LDAP for authentication. In particular LDAP can be a useful technology for synchronizing group membership, while being a security risk for sign in due to the way passwords are handled. Additionally it can allow users to bypass 2FA policies.

Todo

• Test manually to verify sign in page looks right with this disabled
• Add tests
• Create omnibus MR for the gitlab.rb setting

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team