Add a page to display server warnings for administrators #2325

Over time we may deprecate features / configuration fields in the wallet gateway.

We want server admins to be notified of any upcoming changes in a timely manner so it is not a shock when they attempt to upgrade. We should:

1. Log any incompatibilities during startup, AND
2. Add a new page that displays the warnings in the UI, for visibility

The warnings should also disappear after the admin takes a corrective action, i.e. renaming a config field from the deprecated name to the new name, or dropping the deprecated environment variable.