posts: remove unused file path / url methods.

* Remove file_path_for, cropped_file_url (dead code)
* Remove complete_preview_file_url (preview_file_url now returns absolute links)
* Remove `file_name` (only used for Download link in sidebar)

app/views/comments/index.atom.builder

@@ -11,7 +11,7 @@ atom_feed(root_url: comments_url(host: Danbooru.config.hostname)) do |feed|
     feed.entry(comment, published: comment.created_at, updated: comment.updated_at) do |entry|
       entry.title("@#{comment.creator_name} on post ##{comment.post_id} (#{comment.post.humanized_essential_tag_string})")
       entry.content(<<-EOS.strip_heredoc, type: "html")
-        <img src="#{comment.post.complete_preview_file_url}"/>
+        <img src="#{comment.post.preview_file_url}"/>
 
         #{format_text(comment.body)}
       EOS

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

@@ -4,7 +4,7 @@
   <% if CurrentUser.is_member? %>
     <li><%= link_to "Favorite", favorites_path(:post_id => post.id), :remote => true, :method => :post, :id => "add-to-favorites", :title => "Shortcut is F" %></li>
     <li><%= link_to "Unfavorite", favorite_path(post), :remote => true, :method => :delete, :id => "remove-from-favorites" %></li>
-    <li><%= link_to_if post.visible?, "Download", post.file_url, :download => post.presenter.humanized_essential_tag_string + " - " + post.file_name %></li>
+    <li><%= link_to_if post.visible?, "Download", post.file_url, download: post.presenter.filename_for_download %></li>
     <li id="add-to-pool-list"><%= link_to "Add to pool", "#", :id => "pool" %></li>
     <% if post.is_note_locked? %>
       <li id="add-notes-list"><span id="note-locked-notice">Note locked</span></li>