feat: add stacker counter field to the machine

feat: use the stacker amount to properly render the cash cassettes table
feat: add stacker notifications to the plugins backend

lib/machine-loader.js

@@ -35,6 +35,7 @@ function toMachineObject (r) {
       stacker3r: r.stacker3r
     },
     numberOfCassettes: r.number_of_cassettes,
+    numberOfStackers: r.number_of_stackers,
     version: r.version,
     model: r.model,
     pairedAt: new Date(r.created),