Support for anonymous bind to LDAP user management

When retrieving users, groups and memberships from an LDAP server, Confluence binds (authenticates) to LDAP with the configured username and password. It would be good to support anonymous binds to the LDAP server for these operations.

Note that Confluence already binds as a different user when authenticating. The authentication process involves a search for the user with the configured bind DN, binding as the specified user, then performing a search for that user's own data.

The necessary changes are just in the UI. We would need to have a checkbox to enable anonymous binds and permit configurations to leave the bind DN and password blank.