Started working on the frontend

- Added the new post frontend
- Hooked up publishing of posts to the site publisher
- Added an site exporter as a publishing target

views/posts/new.html

@@ -0,0 +1,14 @@
+<main class="flex-grow-1 position-relative">
+  <form action="/sites/{{.site.ID}}/posts" method="post" class="container-fluid post-form p-2">
+    <input type="hidden" name="guid" value="{{ .guid }}">
+    <div class="mb-2">
+      <input type="text" name="title" class="form-control" placeholder="Title">
+    </div>
+    <div>
+      <textarea name="body" class="form-control" rows="3"></textarea>
+    </div>
+    <div>
+      <input type="submit" class="btn btn-primary mt-2" value="Publish">
+    </div>
+  </form>
+</main>