Rename firma to atCompany on server and client

Renames the Zeiterfassung-vs-home flag to a clearer English name
(SQL alias AT_COMPANY, model property AT_COMPANY, JSON atCompany)
in both the server query/model and the client AvailabilityInfo
type and people-tile renderer.

client/src/availability.ts

@@ -73,7 +73,7 @@ export class Availability {
     }
     var location = ''
     if (entry.loggedIn) {
-      location = entry.firma ? ' · Büro' : ' · Home'
+      location = entry.atCompany ? ' · Büro' : ' · Home'
     }
     indicator.innerHTML = '<span class="tapi-dot ' + dotClass + '"></span><small>' + time + location + '</small>'
   }