Is your feature request related to a problem? Please describe.

It isn't obvious when the the CMS is running in local development mode.

Describe the solution you'd like

From #2335, but exact approach is up for discussion:

This could change, but the UI will likely consist of a conspicuous switch in the top right corner, near the user avatar, perhaps showing green when live and red when local. To make local development mode more conspicuous, another indicator like a color bar and notice at screen top may also be added.

This switch will only be visible if local_backend is configured, which will happen by default only for users of the netlify-cms package.

It will probably happen often that a user hits the local development switch and the server isn't running. We should anticipate this by first ensuring the server is reachable when the switch is flipped, and if it isn't, switch back off and provide notification that the local development server isn't reachable.

Additional context

Epic: #3285