base.html: fix HTML syntax
Tag: parents function now return the objects

The function was returning a list of Tag name which is not enough to build the
pages.
Tag: add a box around the images

The size of the box is fixed by the size of the small images. This has to be
replaced by a call to the settings.
Tag: improve the media presentation

The directories have now a fixed size while thumbnails are made of rows of 4
elements.