Add post image upload (#20)

Lands the image upload handlers and picker on master. Questions and replies can attach up to four JPEG, PNG, or WebP photos.

Reviewed-on: #20
Co-authored-by: codegirl-007 <s.raide@gmail.com>

templates/submit.html

@@ -4,7 +4,7 @@
   <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" enctype="multipart/form-data"
     data-submit-once data-submitting-label="Posting…">
     <input type="hidden" name="_csrf" value="{{.CSRF}}">
     <label for="title">Title</label>
@@ -13,6 +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">
+    {{template "imagePicker" (newImagePicker "submit-images")}}
     <button type="submit" class="btn btn-primary" data-submit-button>Submit to today’s hunt</button>
   </form>
 </main>