Always apply GitHub markdown re-formatting features to all objects with markdown

This issue came out of this comment #478851 (comment 2225729213).

Current situation

We have a fragmented approach to re-formatting markdown from GitHub before we import it, which is causing some inconsistencies.

We do two kinds of re-formatting of GitHub markdown:

• Convert full links to the GitHub issues or PRs to full links to the GitLab issues or MRs within MarkdownText.convert_ref_links.
• Convert user mentions to backticks within UsernameMentionRewriter#wrap_mentions_in_backticks

The conversions are both just pure string matching.

But we don't apply both of these in all places where we import objects that have GitHub markdown:

Conversion	Code	Issues	MRs	Notes	Diff notes	Releases	Milestones
Convert full links to the GitHub issues or PRs to full links to the GitLab issues or MRs	MarkdownText.convert_ref_links	⛔	⛔	✅	⛔	⛔	⛔
Convert user mentions to backticks	UsernameMentionRewriter#wrap_mentions_in_backticks	✅	✅	✅	⛔	✅	⛔

Note, we're ignoring the Created by annotations when we didn't map to a user on GitLab, because we'll be removing that soon because of user contribution mapping.

Proposal

• Have a single method that does those conversions. (This might be just extending the existing Gitlab::GithubImport::MarkdownText#format)
• Use that method wherever we import GitHub markdown (see the above table as a starter, but double-check there are not more places).