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>

docs/content/doc/features/authentication.en-us.md

@@ -88,8 +88,8 @@ Adds the following fields:
 - Bind Password (optional)
 
   - The password for the Bind DN specified above, if any. _Note: The password
-    is stored in plaintext at the server. As such, ensure that the Bind DN
-    has as few privileges as possible._
+    is stored encrypted with the SECRET_KEY on the server. It is still recommended
+    to ensure that the Bind DN has as few privileges as possible._
 
 - User Search Base **(required)**