Rewrite ldap autocomplete functionality

[v]

This feature allowed you to type in someone's name/email when you were writing an email

Roundcube has existing functionality for this, but from our understanding, it was not written to handle an LDAP directory as extensive as ours.

We should contribute this to roundcube upstream if this is suitable for contribution.

While rewriting ldap, we should put ldap authentication details in the config/ directory if possible. We want to ensure that ldap config details are not pushed to public repos.

[jmatth]

LDAP mostly works, but we should make matching the middle initial optional.