Refactor mobile chat into modal (#3038)

* feat(mobile): refactor mobile chat into modal

- Make page always scrollable
- Move mobile chat into a standalone modal

* fix(test): split out mobile browser test specs

* fix(mobile): force chat button to render on top of footer

* fix: some small updates from review

* fix: hide/show hide chat menu option based on width

* fix: chat button icon getting cut off

* chore(tests): add browser tests for mobile chat modal

* chore(tests): add story for ChatModal component

* fix(test): quiet shellcheck

* fix: remove unused import

* fix(tests): silence storybook linting warning

* fix(ui): reposition chat modal button icon with transform

web/components/common/UserDropdown/UserDropdown.stories.tsx

@@ -26,7 +26,7 @@ const Example = args => {
     [],
   );
 
-  return <UserDropdown {...args} />;
+  return <UserDropdown id="user-menu" {...args} />;
 };
 
 const Template: ComponentStory<typeof UserDropdown> = args => (