web: indicate that it is in read-only mode

[sudoforge]

right now, there is no visual indication (e.g. a notification in the corner) indicating that the web ui is in read-only mode. we should add support for this.

[MichaelMure]

Eventually that would be like in most apps: showing a login button... but that's when we have external auth

[sudoforge]

yeah, i see in the code that the idea behind the current impl of --read-only mode implies that it's what would be used to support external auth, but:

1. we don't have that in place today, but more critically
2. i think keeping a read-only mode around is actually very useful, to support something like web: run a public deployment of the latest release in --read-only mode #1432 where a consumer wants to expose it publicly, but not allow any interaction at all. so perhaps when we do implement external auth, we keep this around anyway.