tag-browser: assorted improvements to tag browser

This patch patch makes the link generator handle "+" characters in the
filename.
Additionally, file-target list should now have more-correct grid-layout
without producing overflowing rows.
The category list is fixed-positioned sidebar which hopefully makes
mobile rendering work better than before.

index.html

@@ -26,7 +26,7 @@
       <div class="hint placeholder">Select a style category from the left pane</div>
     </div>
     <div>
-      <code-block data-highlight="css" data-matchlinks="[\w-\.]+\.css -> ./chrome/%s" id="previewBox" class="copy-able"></code-block>
+      <code-block data-highlight="css" data-matchlinks="[\w-\.\+]+\.css -> ./chrome/%s" id="previewBox" class="copy-able"></code-block>
     </div>
   </main>
 </div>