media assets: add thumbnail view to /media_assets page.

Add a thumbnail view to the /media_assets page. This page lets you see
all images uploaded to Danbooru by all users (although you can't see who
the uploader is). Also add a link to this page in the subnav bar on the
upload page.

app/components/media_asset_gallery_component.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class MediaAssetGalleryComponent < ApplicationComponent
+  DEFAULT_SIZE = 180
+
+  attr_reader :inline, :size, :options
+
+  renders_many :media_assets, MediaAssetPreviewComponent
+  renders_one :footer
+
+  def initialize(inline: false, size: DEFAULT_SIZE, **options)
+    super
+    @inline = inline
+    @size = size
+    @option = options
+  end
+end