Danger: auto-assign a UX reviewer when a component is being changed

To prevent components from deviating from the design system, we should make UX reviews more systematic in GitLab UI.

Proposal

• Create a mapping of UX designer <-> component so that each component has a stable UX counterpart that can be pinged for reviews.
• Update Danger's setup so that it automatically suggests the correct UX reviewer depending on the component that is being changed.
  • The detection could probably be based on the MR's component::* label. We can expect those labels to be used more frequently once #753 (closed) is addressed.

UX Designer <-> Component mapping

Component	UX designer
component:accordion	@kcomoli, @rayana
component:alert	@andyvolpe
component:avatar	@tauriedavis
component:badge	@pedroms
component:banner	@andyvolpe
component:breadcrumb	@ameliabauerly
component:broadcast-message	@kmann
component:button	@dimitrieh
component:card	@beckalippert
component:chart	@ameliabauerly
~"component:checkbox"	@pedroms
component:data-visualization	@ameliabauerly
~"component:date-picker"	@tauriedavis
component:drawer	@andyvolpe
component:dropdown	@hollyreynolds, @tauriedavis
~"component:file-uploader"	@jareko
~"component:filter"	@matejlatin
component:form	@tauriedavis
component:icon	@jeldergl
component:infinite-scroll	@beckalippert
component:label	@annabeldunstone
component:link	@jeldergl
~"component:list"	@tauriedavis
component:modal	@dimitrieh
component:pagination	@andyvolpe
component:popover	@tauriedavis
component:progress-bar	@jareko
~"component:radio"	@pedroms
~"component:search"	@matejlatin
component:segmented-control	@andyvolpe
component:skeleton-loader	@dimitrieh
component:sorting	@ameliabauerly
component:spinner	@dimitrieh
component:tab	@dimitrieh
component:table	@npost
component:toast	@tauriedavis
component:toggle	@pedroms
~"component:token"	@annabeldunstone
component:tooltip	@rayana
component:tree	@kcomoli, @rayana