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/components/ContextMenu/Template.js

@@ -0,0 +1,169 @@
+// @flow
+import { ExpandedIcon } from "outline-icons";
+import * as React from "react";
+import { useTranslation } from "react-i18next";
+import { Link } from "react-router-dom";
+import {
+  useMenuState,
+  MenuButton,
+  MenuItem as BaseMenuItem,
+} from "reakit/Menu";
+import styled from "styled-components";
+import MenuItem, { MenuAnchor } from "./MenuItem";
+import Separator from "./Separator";
+import ContextMenu from ".";
+
+type TMenuItem =
+  | {|
+      title: React.Node,
+      to: string,
+      visible?: boolean,
+      selected?: boolean,
+      disabled?: boolean,
+    |}
+  | {|
+      title: React.Node,
+      onClick: (event: SyntheticEvent<>) => void | Promise<void>,
+      visible?: boolean,
+      selected?: boolean,
+      disabled?: boolean,
+    |}
+  | {|
+      title: React.Node,
+      href: string,
+      visible?: boolean,
+      selected?: boolean,
+      disabled?: boolean,
+    |}
+  | {|
+      title: React.Node,
+      visible?: boolean,
+      disabled?: boolean,
+      style?: Object,
+      hover?: boolean,
+      items: TMenuItem[],
+    |}
+  | {|
+      type: "separator",
+      visible?: boolean,
+    |}
+  | {|
+      type: "heading",
+      visible?: boolean,
+      title: React.Node,
+    |};
+
+type Props = {|
+  items: TMenuItem[],
+|};
+
+const Disclosure = styled(ExpandedIcon)`
+  transform: rotate(270deg);
+  justify-self: flex-end;
+`;
+
+const Submenu = React.forwardRef(({ templateItems, title, ...rest }, ref) => {
+  const { t } = useTranslation();
+  const menu = useMenuState({ modal: true });
+
+  return (
+    <>
+      <MenuButton ref={ref} {...menu} {...rest}>
+        {(props) => (
+          <MenuAnchor {...props}>
+            {title} <Disclosure color="currentColor" />
+          </MenuAnchor>
+        )}
+      </MenuButton>
+      <ContextMenu {...menu} aria-label={t("Submenu")}>
+        <Template {...menu} items={templateItems} />
+      </ContextMenu>
+    </>
+  );
+});
+
+function Template({ items, ...menu }: Props): React.Node {
+  let filtered = items.filter((item) => item.visible !== false);
+
+  // this block literally just trims unneccessary separators
+  filtered = filtered.reduce((acc, item, index) => {
+    // trim separators from start / end
+    if (item.type === "separator" && index === 0) return acc;
+    if (item.type === "separator" && index === filtered.length - 1) return acc;
+
+    // trim double separators looking ahead / behind
+    const prev = filtered[index - 1];
+    if (prev && prev.type === "separator" && item.type === "separator")
+      return acc;
+
+    // otherwise, continue
+    return [...acc, item];
+  }, []);
+
+  return filtered.map((item, index) => {
+    if (item.to) {
+      return (
+        <MenuItem
+          as={Link}
+          to={item.to}
+          key={index}
+          disabled={item.disabled}
+          selected={item.selected}
+          {...menu}
+        >
+          {item.title}
+        </MenuItem>
+      );
+    }
+
+    if (item.href) {
+      return (
+        <MenuItem
+          href={item.href}
+          key={index}
+          disabled={item.disabled}
+          selected={item.selected}
+          target="_blank"
+          {...menu}
+        >
+          {item.title}
+        </MenuItem>
+      );
+    }
+
+    if (item.onClick) {
+      return (
+        <MenuItem
+          as="button"
+          onClick={item.onClick}
+          disabled={item.disabled}
+          selected={item.selected}
+          key={index}
+          {...menu}
+        >
+          {item.title}
+        </MenuItem>
+      );
+    }
+
+    if (item.items) {
+      return (
+        <BaseMenuItem
+          key={index}
+          as={Submenu}
+          templateItems={item.items}
+          title={item.title}
+          {...menu}
+        />
+      );
+    }
+
+    if (item.type === "separator") {
+      return <Separator key={index} />;
+    }
+
+    return null;
+  });
+}
+
+export default React.memo<Props>(Template);