Mark the plan field as required in form

Browser will fortunately validate this and we won't have invalid
(undefined) plans sent.

Author: pgiraud

app/templates/index.html

@@ -31,7 +31,7 @@ <h2>
         </div>
         <div class="mb-3">
           <label for="planInput" class="form-label">Plan <span class="small text-muted">(text or JSON)</span></label>
-          <textarea name="plan" class="form-control" :class="[draggingPlan ? 'dropzone-over' : '']" id="planInput" rows="8" v-model="planInput" @dragenter="draggingPlan = true" @dragleave="draggingPlan = false" @drop.prevent="handleDrop" placeholder="Paste execution plan or drop a file"></textarea>
+          <textarea name="plan" class="form-control" :class="[draggingPlan ? 'dropzone-over' : '']" id="planInput" rows="8" v-model="planInput" @dragenter="draggingPlan = true" @dragleave="draggingPlan = false" @drop.prevent="handleDrop" placeholder="Paste execution plan or drop a file" required></textarea>
         </div>
         <div class="mb-3">
           <label for="queryInput" class="form-label">Query <span class="small text-muted">(optional)</span></label>