Improve support for external issue references

Merged username-removed-378947 requested to merge adam-external-issue-references-spike into master

What does this MR do?

  1. Fixes referenced bugs related to extracting references to external issues
  2. Cleans up the code - Banzai::Filter::IssueReferenceFilter should not concerned with external issues, because we have Banzai::Filter::ExternalIssueReferenceFilter.
  3. Clarifies in the documentation what we support and what we do not support

Are there points in the code the reviewer needs to double check?

General caution, this code was very messy and unintuitive, and it still is.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #33679 (closed)

Closes #34076 (closed)

Closes #34082 (closed)

Edited by username-removed-378947