LDAP: finishing touches (#17945)

* LDAP:Docs: `active_sync_enabled` setting

Mention `active_sync_enabled` setting and enable it by default

* LDAP: move "disableExternalUser" method

Idea behind new design of the LDAP module is to minimise conflation
between other parts of the system, so it would decoupled as much as
possible from stuff like database, HTTP transport and etc.

Following "Do One Thing and Do It Well" Unix philosophy principal, other things
could be better fitted on the consumer side of things.

Which what this commit trying to archive

* LDAP: correct user/admin binding

The second binding was not happening, so if the admin login/password
in LDAP configuration was correct, anyone could had login as anyone using
incorrect password

docs/sources/auth/enhanced_ldap.md

@@ -44,7 +44,7 @@ a user as member of a team and it will not be removed when the user signs in. Th
 
 ## Active LDAP Synchronization
 
->  Only available in Grafana Enterprise v6.3+
+> Only available in Grafana Enterprise v6.3+
 
 In the open source version of Grafana, user data from LDAP will be synchronized only during the login process when authenticating using LDAP.
 
@@ -60,8 +60,11 @@ With this feature you can configure Grafana to actively sync users with LDAP ser
 # @weekly                | Run once a week, midnight between Sat/Sun  | 0 0 0 * * 0
 # @daily (or @midnight)  | Run once a day, midnight                   | 0 0 0 * * *
 # @hourly                | Run once an hour, beginning of hour        | 0 0 * * * *
-sync_cron = "@hourly"
+sync_cron = "0 0 1 * * *" # This is default value (At 1 am every day)
 # This cron expression format uses 6 space-separated fields (including seconds), for example
 # sync_cron = "* */10 * * * *"
 # This will run the LDAP Synchronization every 10th minute, which is also the minimal interval between the grafana sync times i.e. you cannot set it for every 9th minute
+
+# You can also disable active LDAP synchronization
+active_sync_enabled = true # enabled by default
 ```