Every object processed by ExternalIssueReferenceFilter can return a
different Project instance when calling "project". For example, every
note processed will have it's own associated Project. If we were to
cache Project#default_issues_tracker? on Project level this would have
no impact on Markdown rendering timings as the cache would have to be
built for every Project instance without it ever being re-used.

To work around this we cache Project#default_issues_tracker? in
Banzai::Filter::ExternalIssueReferenceFilter using the project's _id_
instead of the whole object. This setup allows re-using of the cached
data even when the Project instances used are different, as long as the
actual project IDs are the same.