Remove “Jump to next unresolved thread” button in merge request threads

Related to #16798 (closed) and the discussion in its MR !23123 (comment 273084065).

Look at the tracking data in 4 milestones after #199458 (closed) is shipped. Revisit our decision of keeping both buttons and decide if we remove the “Jump to next unresolved thread” button present in each thread. If we decide to remove it, consider a popover that points at the global “Jump to…” button at the top of merge requests.

Proposal

Given that the usage of this button on GitLab.com is very low, let's remove it behind a feature flag. Once it's shipped we can turn the feature flag on in phases, giving enough time to listen to user feedback:

1. On for the GitLab.org group
2. Then on for GitLab.com
3. And then finally make it the default for all instances