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/computers/frontend/views/PCForm.vue

@@ -149,7 +149,7 @@
                 :key="l.locationid"
                 :value="l.locationid"
               >
-                {{ l.location }}
+                {{ l.locationname }}
               </option>
             </select>
           </div>