This applies the changes introduced in `Project.public_or_visible_to_user`
to the snippets finder only.

We know that for `SnippetsFinder`, this change improves SQL timing from
5/23/25s to 0.7/2/4s (mean/p95/p99). At the same time, the scope was too
broad, (negatively) affecting SQL timings in various other places:

https://gitlab.com/gitlab-com/infrastructure/issues/3772

With this commit, the snippets dashboard stays usuable as we generally
don't run into statement timeouts. In contrast to the earlier change in
!17088, the scope of the change is limited to `SnippetsFinder` only,
thus not affecting other places.

These changes were introduced in MR
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17088. In
https://gitlab.com/gitlab-com/infrastructure/issues/3772 we discovered
these changes lead to a pretty drastic increase in SQL response timings.

We'll revert these changes so we can work on a better solution in the
mean time without GitLab.com (or other installations) experiecing
reduced performance in the mean time.

Immediately using #from here requires a lot of changes in
other finders (e.g. IssuableFinder, TodosFinder). In all places where we
use #merge, this goes completely the wrong way when passed in a relation
that was built with `#from(...)`: The original query's FROM part gets
completely replaced.

This avoids changing all other places and focuses on improving
SnippetFinder with the downside of two (small) codepaths to do the same
thing.

Merge branch 'security-10-4-25223-snippets-finder-doesnt-obey-feature-visibility' into 'security-10-4'

[Port for security-10-4]: Makes SnippetFinder ensure feature visibility

Closes #40755.

Refactor snippets finder & dont return internal snippets for external users

See merge request !2094

Adding the necessary API for the new /snippets Restful resource
added with this commit. Added a new Grape class `Snippets`, as
well as a `PersonalSnippet` entity.

Issue: #20042
Merge-Request: !6373
Signed-off-by: NGuyzmo <guyzmo+gitlab+pub@m0g.net>

This reverts commit 13e37a3e.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15580

This was removed from the interface in
https://github.com/gitlabhq/gitlabhq/pull/6027 but its implementation
lingered around for two years.