Add a checkbox to the LDAP auth configuration UI to "Always Search"

Summary: Fixes T3208. This forces us to bind+search even if there are no anonymous credentials.

Test Plan: Checked the box, saved the form. Unchecked the box, saved the form. LDAP??

Reviewers: Firehed

Reviewed By: Firehed

Subscribers: epriestley

Maniphest Tasks: T3208

Differential Revision: https://secure.phabricator.com/D8723