feat: file mobile issues without an owner (Closes #855)

frontend/index.html

@@ -806,6 +806,7 @@
       <label class="create-issue-owner" for="create-issue-assignee">Owner
         <select id="create-issue-assignee" aria-describedby="create-issue-assignee-status">
           <option value="">Me</option>
+          <option value="__unassigned__">No owner</option>
         </select>
         <span id="create-issue-assignee-status" class="small" aria-live="polite">Open the owner picker to load eligible teammates.</span>
       </label>