LDAP synchronization should warn before it kills direct member list

Issue

Opening as feature as per discussion in https://federal-support.gitlab.com/hc/en-us/requests/868

Scenario: we have an existing gitlab self-managed server with LDAP configured. We have an existing gitlab "group" configured with direct members. We decide to add additional users to the gitlab "group" via an LDAP configuration within the GUI. We add an LDAP group to the gitlab group with "maintainer" access and synchronize. All the existing direct members outside of the LDAP group lose their access!

It would be nice if the GUI provided a warning about the direct members being lost before applying in this scenario.

Note that this is the documented behavior. However, from a UI/UX perspective, it's usually a good idea for GUIs to warn about dangerous actions. This is an example of the "principle of least surprise".

Proposal

Add a warning message

Synchronizing groups using LDAP causes all users outside of the LDAP group to lose their permissions. Are you sure you want to continue?

User will have the option to continue or to cancel this action

Visuals

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.