Disable Sharing of Groups Outside Hierarchy PoC

What does this MR do?

Proof of concept for #217623 (closed)

Screenshots

With the feature off (default, current behavior)

The group can be shared with groups outside the group hierarchy.

In this case, the Mars group is shared with the Books group, which is outside the Solar System group hierarchy.

Screen_Recording_2021-05-26_at_6.28.26_PM

With the feature turned on

Turn the feature on

TurnOnFeature

Sharing Outside Group Hierarchy is Disabled

The group cannot be shared with groups that lie outside the Solar System group hierarchy.

Notice the Books and Animals groups, visible in the previous scenario, are not options in the group search.

The Mars group can still be shared with any groups within the Solar System hierarchy.

CannotShare

The Setting Appears for a Root Group

The Setting Does Not Appear for a Subgroup

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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