GitLab LDAP Group Sync "Dry Run" functionality

Proposal

There are frequent support tickets being submitted when users are trying to use LDAP Group Sync functionality.

This includes the Sync button on a Group's Members page being "stuck" (docs link) for various reasons, an unexpected number of users being removed (or added) to a group and other similar issues.

I believe that we could minimise this by giving customers the ability to "test" what the effect of adding a Group Sync would be.

When specifying an LDAP Group Link, we could run the sync and display what the results of the sync would be and then give the customer the ability to save the change:

Time to response from LDAP server: 3000ms
New members added: 3
Existing members removed: 0

OR

LDAP server responded with error: "Error Message"

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Another option would be to create a rake task that would output something like the above.