Resolve "Discussion counter is wrong on issue page"

What does this MR do?

Resolves the situation where the Discussion tab counter would sometimes reflect the wrong count.

We were relying on the list of allDiscussions but it's more complicated than that.

We have to filter out system notes and count all individual notes in threads.

Plus, we already have a getter for that.

Screenshots

Will add soon.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Addresses #13370 (closed)