Refactor html templates

This commit entirely refactors the use of html templates. Instead of
inheriting from a shared template, we insert the results into a base
template.

pkg/httpx/form.html

@@ -0,0 +1,22 @@
+<div class="pure-u-1">
+    {{ block "form/extra" . }}<!-- no extra -->{{ end }}
+
+    <form class="pure-form pure-form-aligned" method="POST">
+        <fieldset>
+            {{ block "form/message" . }}
+                {{ $E := .Error }}
+                {{ if not (eq $E "") }}
+                    <div class="pure-form-group">
+                        <p class="error-message">
+                            {{ $E }}
+                        </p>
+                    </div>
+                {{ end }}
+            {{ end }}
+
+            {{ block "form/inside" . }}<!-- no inside -->{{ end }}
+            {{ .Form }}
+            <input type="submit" value="{{ block "form/button" .}}Submit{{ end }}" class="pure-button">
+        </fieldset>
+    </form>
+</div>