fix: Keyboard accessible context menus (#1768)

- Makes menus fully accessible and keyboard driven
- Currently adds 2.8% to initial bundle size due to the inclusion of Reakit and its dependency, popperjs.
- Converts all menus to functional components
- Remove old custom menu system
- Various layout and flow improvements around the menus

closes #1766

.flowconfig

@@ -32,6 +32,7 @@ module.file_ext=.json
 esproposal.decorators=ignore
 esproposal.class_static_fields=enable
 esproposal.class_instance_fields=enable
+esproposal.optional_chaining=enable
 
 suppress_comment=\\(.\\|\n\\)*\\$FlowFixMe
 suppress_comment=\\(.\\|\n\\)*\\$FlowIssue