Encrypt LDAP bind password in db with SECRET_KEY (#15547)

* Encrypt LDAP bind password in db with SECRET_KEY

The LDAP source bind password are currently stored in plaintext in the db
This PR simply encrypts them with the setting.SECRET_KEY.

Fix #15460

Signed-off-by: Andrew Thornton <art27@cantab.net>

* remove ui warning regarding unencrypted password

Co-authored-by: silverwind <me@silverwind.io>

options/locale/locale_en-US.ini

@@ -2283,7 +2283,6 @@ auths.host = Host
 auths.port = Port
 auths.bind_dn = Bind DN
 auths.bind_password = Bind Password
-auths.bind_password_helper = Warning: This password is stored in plain text. Use a read-only account if possible.
 auths.user_base = User Search Base
 auths.user_dn = User DN
 auths.attribute_username = Username Attribute