Wiki preview URL converting problem [via Markdown]

Current implementation when rendering the preview, thinks relative links are for project repository files.

We are creating a new preview route that will define correct context data to render for wikis instead.

Fixes #2380, #1184

See merge request !3461

This fixes the problem where Markdown such as:

    ### 31st

Would get rendered as a link tag pointing to issue number 31 inside a
header tag.

See gitlab-org/gitlab-ce#14936 for more information.

Resolves #14411.

See merge request !3464

Cleaning this up any further is a bit tricky as the caches in question
should only be evaluated if RequestStore is actually enabled.

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.