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

app/menus/MemberMenu.js

@@ -0,0 +1,36 @@
+// @flow
+import { observer } from "mobx-react";
+import * as React from "react";
+import { useTranslation } from "react-i18next";
+import { useMenuState } from "reakit/Menu";
+import ContextMenu from "components/ContextMenu";
+import OverflowMenuButton from "components/ContextMenu/OverflowMenuButton";
+import Template from "components/ContextMenu/Template";
+
+type Props = {|
+  onRemove: () => void,
+|};
+
+function MemberMenu({ onRemove }: Props) {
+  const { t } = useTranslation();
+  const menu = useMenuState({ modal: true });
+
+  return (
+    <>
+      <OverflowMenuButton {...menu} />
+      <ContextMenu {...menu} aria-label={t("Member options")}>
+        <Template
+          {...menu}
+          items={[
+            {
+              title: t("Remove"),
+              onClick: onRemove,
+            },
+          ]}
+        />
+      </ContextMenu>
+    </>
+  );
+}
+
+export default observer(MemberMenu);