Warn users about web app limitations on mobile browsers

As Monaco Editor does not support mobile browsers and the user will not have the best experience using the web version of Simplenote, It would be good if the application could warn them, letting them know about the limitations and proposing them to install the native app, as @pachlava suggested.

### What
Warn the users on mobile browsers about the known limitations and suggest them to install the app.

### Why
Experience for them will not be the best and can vary from not being able to interact with a checkbox to duplicating the last word while trying to add one more character as shown in the video below.

https://user-images.githubusercontent.com/42008628/132212174-0460e564-3815-46a1-ae9b-bbba27d31d8f.mp4

### How
Notification bar or a banner displayed based on user agent.

No files, tests, or entry points are named. Start by locating the web application's browser or user-agent detection and the existing notification or banner UI, then check how native-app installation links are presented. Done means mobile-browser users see a clear limitations warning and a suggestion to install the native app, without affecting supported browsers.

Written by the indexing model from the issue text.