/media_assets: add basic index and show pages.

* Add a basic index page at https://danbooru.donmai.us/media_assets.
* Add a basic show page at https://danbooru.donmai.us/media_assets/1.
* Add ability to search /media_assets.json by metadata. Example:
** https://danbooru.donmai.us/media_assets.json?search[metadata][File:ColorComponents]=3
* Add a "»" link next to the filesize on posts linking to the metadata page.

Known issues:

* Sometimes the MD5 links on the /media_assets page return "That record
  was not found" errors. These are unfinished uploads that haven't been
  made into posts yet.
* No good way to search for custom metadata fields in the search form.
* Design is ugly.

app/views/posts/partials/show/_information.html.erb

@@ -16,6 +16,7 @@
   <li id="post-info-size">
     Size: <%= link_to_if policy(post).visible?, "#{number_to_human_size(post.file_size)} .#{post.file_ext}", post.tagged_file_url %>
     (<%= post.image_width %>x<%= post.image_height %>)
+    <%= link_to "»", @post.media_asset %>
   </li>
   <li id="post-info-source">Source: <%= post_source_tag(post.source, post.normalized_source) %></li>
   <li id="post-info-rating">Rating: <%= post.pretty_rating %></li>