Include all available quick actions in autocomplete

this is a follow up from #290988 (comment 462045190)

Problem

When you type / in a comment or description, we only show 5 quick actions by default:

This makes the other quick actions not easily discoverable as I would have to open our documentation to view them all. We also want to work towards removing this:

From every comment box. This would also help offset the discoverability problem if we were to move forward with that.

Proposal

Show all Quick Actions as a scrollable list in the autocomplete modal.

Release notes

All available Quick Actions are now displayed in a scrollable list when / is entered into a description or comment field. Quick Actions are a great way to efficiently update data on an issue, merge request, or epic and this small improvement helps them be more discoverable directly within the context where they are used.