Proposal: ignore strings in backticks in Danger review

What does this MR do?

Changes the regex used by Danger review so that strings like `#1234567` do not trigger the "don't use short references to GitLab entities" rule (but strings like #1234567 still do)

With this change, any string enclosed in backticks in a commit message will not be considered a match for GitLab issue/epic/MR/Milestone references. This means that hex color values can be referenced in a commit message without a false positive failure from Danger, as long as you wrap them in backticks.

Here's an example commit message that currently fails, but would pass with this change:

feat(GlModal)

Change modal text from `#000` to `$black`

Commits like these would still fail:

feat(GlModal)

Make all the corners razor sharp

Cf. related epic &241213

feat(GlModal)

Address comments in gitlab#241213

Related to gitlab#452225 (closed)