Add Keyboard Shortcuts Legend (#5823)

* Add Keyboard Shortcuts Legend

Adds a "Keyboard Shortcuts" legend (displayed in the rightmost column)
which is toggled via a new "?" hotkey. When subsequently pressed from
the Keyboard Shortcuts legend, "?" will navigate back to the previous
location.

* Add hidden table headings.

Makes the headings available for accessibility but hides them visually.

app/javascript/mastodon/features/ui/util/async-components.js

@@ -38,6 +38,10 @@ export function GettingStarted () {
   return import(/* webpackChunkName: "features/getting_started" */'../../getting_started');
 }
 
+export function KeyboardShortcuts () {
+  return import(/* webpackChunkName: "features/keyboard_shortcuts" */'../../keyboard_shortcuts');
+}
+
 export function PinnedStatuses () {
   return import(/* webpackChunkName: "features/pinned_statuses" */'../../pinned_statuses');
 }