ci(danger): UX reviewers for component changes

What does this MR do?

This MR modifies Danger's setup so that it suggests UX reviewers when an MR changes one or more components.

The new rule is based on labels. When Danger detects that one of the ~component:* label was set on an MR, it will assign one of the corresponding UX reviewers as defined in the issue.

Here's how the Danger report would look like for an MR that has the component:pagination and component:progress-bar labels:

Reviewer roulette preview

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.

Closes #757 (closed)