Add `server` package

[weasdown]

Add server package that adds the following functionality:

• Move app code to a new app folder.
• Pre-Alert server. Pre-Alerts are Events that have the EventStatus (to be written) preAlert, so have not yet been assigned a NOC.
• Event server. Supplies the list of current Events, including which Unit(s) are assigned to each Event.
• Unit server. This shows which Units are currently logged in and the UnitStatus (#11) for each.

The functionality above may be implemented as separate servers, but more likely as different "routes" as per the server in Flutter's compass_app example app.