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.
This patch makes tag-browser layout somewhat usable on mobile.
Additionally it adds few banner hints to use Ctrl keys and basic hint to
select a category when empty.
This also makes the Copy-button hidden until some content is shown in
code-block.
