fix: location dropdowns rendered blank (wrong field) + require printer model

The location option label read l.location, but the Location.to_dict() field is
locationname, so every option rendered blank - the dropdown looked empty and
"massive" (a long list of blank rows). Fixed across all five affected forms:
printers, computers, network devices, network device form, and the subnets
location filter. Other .location uses (printer-driver URL, search-result label,
report bylocation key) are legitimately different fields, left alone.

Also require a model on the printer form: asterisk + required attr, plus a JS
guard in savePrinter (the native required is skipped while the select is
disabled with no vendor picked) that points the user at the vendor first.

plugins/network/frontend/views/NetworkDeviceForm.vue

@@ -81,7 +81,7 @@
               <select id="locationid" v-model="form.locationid" class="form-control">
                 <option value="">Select Location</option>
                 <option v-for="loc in locations" :key="loc.locationid" :value="loc.locationid">
-                  {{ loc.location }}
+                  {{ loc.locationname }}
                 </option>
               </select>
             </div>

plugins/network/frontend/views/NetworkDevicesList.vue

@@ -42,7 +42,7 @@
       <select v-model="locationFilter" class="form-control" @change="loadDevices">
         <option value="">All Locations</option>
         <option v-for="loc in locations" :key="loc.locationid" :value="loc.locationid">
-          {{ loc.location }}
+          {{ loc.locationname }}
         </option>
       </select>
     </div>

plugins/network/frontend/views/SubnetsList.vue

@@ -148,7 +148,7 @@
                   <select id="locationid" v-model="form.locationid" class="form-control">
                     <option value="">Select Location</option>
                     <option v-for="loc in locations" :key="loc.locationid" :value="loc.locationid">
-                      {{ loc.location }}
+                      {{ loc.locationname }}
                     </option>
                   </select>
                 </div>