New "warning" text field in chain configs

[katspaugh]

Is your feature request related to a problem? Please describe.
We sometimes show a green banner on top of the website when some service is down. This is currently done via Beamer, which is a 3rd party service that we load only if the user agreed to store "support & updates" cookies, which is not the default.

Describe the solution you'd like
It would be nice to show this banner natively instead, via the website itself.

I suggest to add a new text field per each chain config, e.g. warning, that the app would read and render as a banner for that chain.

This is not urgent, just a potential improvement when we have spare time

[hectorgomezv]

Open for discussion here

[katspaugh]

Reopened to track it, it's still important to have a way to communicate annoucnement to users w/o a cookie-gated 3rd-party service.