When a request contains an ETag value in its If-None-Match header, the backend
may send a request ID (from Rack) that does not correspond to a value in Peek's
Redis cache (because we aborted the Rails processing in our ETag middleware).

Because a cached response (304) has to replace the headers with those from the
previous 200 - https://tools.ietf.org/html/rfc7234#section-4.3.4 - we add a
custom header that will only be present in cache hits, that can tell the
frontend to ignore these.

Closes #45412

This is useful for a transition period to migrate away from
`NoninternalAtomicId`. In a situation where both the old and new code to
generate a iid value is run at the same time (for example, during a
deploy different nodes may serve both versions), this will lead to
problems regarding the correct `last_value`. That is, what we track in
`InternalId` may get out of sync with the maximum iid present for
issues.

With this change, we double-check that and correct the `last_value` with
the maximum iid found in issues if necessary.

This is subject to be removed with the 10.8 release and tracked over
here: https://gitlab.com/gitlab-org/gitlab-ce/issues/45389

Closes #45269.

[ci skip] Adds specs

Adds specs

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

If form does not have import sources checkboxes we should not reset
import sources to empty. This fixes issue when import sources got reset
after user modifies unrelated settings section like GitLab pages

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes #45326

Closes #45314

Revert "Merge branch '44870-remove-extra-space-around-comment-form-on-merge-requests' into 'master'"

This reverts merge request !18120

Some specs fail in rails5 branch with errors like

```
1) Group#secret_variables_for when the ref is not protected contains only the secret variables
    Failure/Error: variables = Ci::GroupVariable.where(group: list_of_ids)

    NoMethodError:
      undefined method `arel_table' for Ci::Group:Class
```

This commit fixes it.