Setup LDAP override confirmation modal [RUN AS-IF-FOSS]

What does this MR do?

Part of &4233 (closed) and #238453 (closed)

As part of an effort to convert the group members page from HAML to Vue this adds part of the functionality needed to allow overriding members added with LDAP group sync. This MR adds the confirmation modal and Vuex actions needed for this functionality. In an effort to keep MRs as small as possible the button that triggers the modal and the dropdown item that allows you to disable LDAP override will come in subsequent MRs.

Local testing

Currently this is behind the vue_group_members_list feature flag. To enable:

1. bin/rails console
2. Feature.enable(:vue_group_members_list)

Setting up LDAP with GDK is a bit tricky, but it can be done using the GDK LDAP guide

Since the button that triggers the modal doesn't exist yet this is probably the easiest way to test for now:

Screenshots

I struggled a little bit with the best way to split these MRs up. The below video includes some functionality that will be added in subsequent MRs.

New members view

Screen_Recording_2020-10-19_at_10.10.44_PM

Old members view

Screen_Recording_2020-10-19_at_10.22.23_PM

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind a feature flag
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team