media assets: move more file-handling logic into MediaAsset.

Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.

app/logical/storage_manager.rb

@@ -109,13 +109,13 @@ class StorageManager
 
     case type
     when :preview
-      "#{base_dir}/preview/#{subdir}#{file}"
+      "/preview/#{subdir}#{file}"
     when :crop
-      "#{base_dir}/crop/#{subdir}#{file}"
+      "/crop/#{subdir}#{file}"
     when :large
-      "#{base_dir}/sample/#{subdir}#{file}"
+      "/sample/#{subdir}#{file}"
     when :original
-      "#{base_dir}/original/#{subdir}#{file}"
+      "/original/#{subdir}#{file}"
     end
   end
 
@@ -145,4 +145,8 @@ class StorageManager
     tags = post.presenter.humanized_essential_tag_string.gsub(/[^a-z0-9]+/, "_").gsub(/(?:^_+)|(?:_+$)/, "").gsub(/_{2,}/, "_")
     "__#{tags}__"
   end
+
+  def full_path(path)
+    File.join(base_dir, path)
+  end
 end