/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/media_assets/index.html.erb

@@ -0,0 +1,39 @@
+<div id="c-media-assets">
+  <div id="a-index">
+    <%= search_form_for(media_assets_path) do |f| %>
+      <%= f.input :md5, input_html: { value: params[:search][:md5] } %>
+      <%= f.input :image_width, input_html: { value: params[:search][:image_width] } %>
+      <%= f.input :image_height, input_html: { value: params[:search][:image_height] } %>
+      <%= f.input :file_size, input_html: { value: params[:search][:file_size] } %>
+      <%= f.input :file_ext, input_html: { value: params[:search][:file_ext] } %>
+
+      <%= f.simple_fields_for :metadata do |meta| %>
+        <% params.dig(:search, :metadata).to_h.each do |key, value| %>
+          <%= meta.input key, label: key, input_html: { value: value } %>
+        <% end %>
+      <% end %>
+
+      <%= f.submit "Search" %>
+    <% end %>
+
+    <%= table_for @media_assets, class: "striped autofit" do |t| %>
+      <% t.column "MD5", td: { class: "font-monospace" } do |media_asset| %>
+        <%= link_to media_asset.md5, posts_path(md5: media_asset.md5) %>
+      <% end %>
+      <% t.column :image_width %>
+      <% t.column :image_height %>
+      <% t.column :file_size %>
+      <% t.column :file_ext %>
+
+      <% t.column "Metadata" do |media_asset| %>
+        <%= link_to pluralize(media_asset.metadata.size, "tags"), media_asset %>
+      <% end %>
+
+      <% t.column "Created" do |media_asset| %>
+        <%= time_ago_in_words_tagged(media_asset.created_at) %>
+      <% end %>
+    <% end %>
+
+    <%= numbered_paginator(@media_assets) %>
+  </div>
+</div>

app/views/media_assets/show.html.erb

@@ -0,0 +1,20 @@
+<div id="c-media-assets">
+  <div id="a-show" class="fixed-width-container">
+    <h1 class="mb-4">Media Asset</h1>
+
+    <% if @post.present? %>
+      <%= post_preview(@post, show_deleted: true) %>
+    <% else %>
+      <p>MD5: <%= link_to @media_asset.md5, posts_path(md5: @media_asset.md5) %></p>
+    <% end %>
+
+    <table class="striped aligned-vertical">
+      <% @media_asset.metadata.sort.each do |key, value| %>
+        <tr>
+          <th><%= key %></th>
+          <td><%= link_to value, media_assets_path(search: { metadata: { key => value }}) %></td>
+        </tr>
+      <% end %>
+    </table>
+  </div>
+</div>

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>