Livebook app servers/offline should show more prominent warnings

[josevalim]

Currently we show it only on the workspace but that may not be clear enough. My suggestion is to get these two messages, store them in Livebook.Config, and make it visible on all "admin" pages (home, settings, workspaces, etc):

livebook/lib/livebook/hubs/team.ex

Lines 145 to 150 in 0ae9c18

	team.offline ->
	"You are running an offline Workspace for deployment. You cannot modify its settings."
	team.user_id == nil ->
	"You are running a Livebook app server. This worksace is in read-only mode."

This should help avoid confusion as in #2826.

[josevalim]

The message should be a bit more general and say:

This Livebook instance has been configured for notebook deployment and is in read-only mode.