closes https://linear.app/tryghost/issue/ENG-739
- `props.innerRef` in `<IFrame>` was inadvertently assumed to always exist, sometimes throwing an error on render when certain popups like the profile settings modal were opened resulting in the app crashing and the whole comments block disappearing
- added a guard to ensure the ref exists before trying to set it
- updated tests so the profile modal route is tested
refs https://github.com/TryGhost/Product/issues/3504
- App component now uses React hooks intead of React class component
- App is now written in TypeScript
- All JavaScript is now removed from the Comments-UI project
- Removed `PopupNotification` because these were never displayed
- Removed `action` from AppContext (never used)
- Moved options parsing out of `index.ts` into a separate utility file,
similar to the signup-form
- Improved reliability of some editor tests by always waiting for the
editor to be focused (was not always the case) + added an utility method
for this
