The RelativePositioning::Range class is reponsible for inferring
the other end of ranges, testing if the range is open and detecting
if an object is contained within the range (allowing us to avoid some
operations).

This divides up the relative_positioning class specs into the
appropriate places in the lib tree

This moves Mover, ItemContext and Gap into the lib tree where they can
be used separately from the app code.

The changes tested here are to see if in EE projects in the same groups
count as part of the same relative positioning set. We do not need to
test every method - just the scoping query.

Frankly a bunch of the movement tests could also be removed, but they
are all calling public methods so it is fine for them to test the
interface.

This is now part of the usage instructions in the relative positioning
shared examples.

Since classes that include RelativePositioning no longer can override
low-level implementation methods to hook into internal details, this
means that catching query timeouts and other failures systematically is
no longer straightforward.

This change allows classes to implement a new callback hook:
`#could_not_move`, which is called with an exception if any of the
movement methods times out.

This involves custom handling for polymorphic models, and null-safety.

Hetrogenous sets of items need to override a couple of methods

These are tested by their behavior

By testing this, we can remove the tests on private members.

this comes at the cost of less-than-correct behaviour

See: https://gitlab.com/gitlab-org/gitlab/-/issues/238416

[Relative Positioning] Correct sibling query for epic tree nodes

See merge request gitlab-org/gitlab!41986

Expose dependency path

See merge request gitlab-org/gitlab!40644

Add mock data

Add docs for custom schema validation

See merge request gitlab-org/gitlab!42194

Outside the Syntax highlighting header as per:
https://gitlab.com/gitlab-org/gitlab/-/issues/247483#note_413000624

Track projects using code intelligence

See merge request gitlab-org/gitlab!41881

Don't apply Ctrl markdown shortcuts on Mac

See merge request gitlab-org/gitlab!42239

Update buttons in the Add issues modal in Boards

Closes #229321

See merge request gitlab-org/gitlab!42317

Add note for red merge button to MR docs

Closes #245329 and #245326

See merge request gitlab-org/gitlab!41608

Add tracking to group invite banner

Closes #219240 and #229619

See merge request gitlab-org/gitlab!41774

Projects::CreateService to consider only group membership for admin

See merge request gitlab-org/gitlab!42335

Remove remaining deprecated GlNew references

See merge request gitlab-org/gitlab!42422

Don't clean Sidekiq processes when fetching metrics

See merge request gitlab-org/gitlab!42463

Docs: Clarify definition of subject

See merge request gitlab-org/gitlab!42445

Add *_VERSION files to the .backend-patterns list

Closes #250272

See merge request gitlab-org/gitlab!42444

Display Contributor and Author badges on notes

Closes #16534

See merge request gitlab-org/gitlab!40198

Add instance level variables to the priority list

See merge request gitlab-org/gitlab!42442

docs: Fix typo in GitLab Notification Email

See merge request gitlab-org/gitlab!42447

Fix Gitaly update Slack notification

See merge request gitlab-org/gitlab!42369