Add controls for LDAP group sync [RUN AS-IF-FOSS]

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

As part of an effort to convert the group members page from HAML to Vue this adds the functionality needed to allow overriding members added with LDAP group sync.

UX already reviewed in !45612 (comment 432412043)

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

Screenshots

New group members view

Screen_Recording_2020-10-21_at_11.44.30_PM

Current group members view

Screen_Recording_2020-10-21_at_11.32.16_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