Prevent duplicate post submissions

templates/submit.html

@@ -4,7 +4,8 @@
   <h1>Ask a question</h1>
   <p class="lede">It lands on today’s hunt (Pacific time). People vote; the ranking resets at midnight PT.</p>
   {{if .Error}}<p class="banner error" role="alert">{{.Error}}</p>{{end}}
-  <form class="ask" method="post" action="/submit">
+  <form class="ask" method="post" action="/submit"
+    data-submit-once data-submitting-label="Posting…">
     <input type="hidden" name="_csrf" value="{{.CSRF}}">
     <label for="title">Title</label>
     <input id="title" name="title" type="text" required maxlength="120" value="{{.TitleVal}}" placeholder="Water heater popping after showers">
@@ -12,7 +13,7 @@
     <textarea id="body" name="body" rows="8" required maxlength="8000" placeholder="Age of the house, what you already tried, where you are in the Bay if it helps.">{{.BodyVal}}</textarea>
     <label for="city">City <span class="optional">(optional)</span></label>
     <input id="city" name="city" type="text" maxlength="80" value="{{.CityVal}}" placeholder="Oakland">
-    <button type="submit" class="btn btn-primary">Submit to today’s hunt</button>
+    <button type="submit" class="btn btn-primary" data-submit-button>Submit to today’s hunt</button>
   </form>
 </main>
 {{template "footer" .}}