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.

internal/dis/component/server/admin/html/instance.html

@@ -1,7 +1,3 @@
-{{ template "_base.html" . }}
-{{ define "title" }}{{ .Instance.Slug }}{{ end }}
-
-{{ define "content" }}
 <div class="pure-u-1-1">
     <h2 id="overview">Info &amp; Status</h2>
 </div>
@@ -484,5 +480,4 @@
             <button class="remote-action pure-button pure-button-danger" data-action="purge" data-param="{{ .Instance.Slug }}" data-confirm-param="#purge-confirm-slug" data-buffer="1000" data-force-reload="/admin/">Purge Instance</button>
         </fieldset>
     </form>
-</div>
-{{ end }}
+</div>