The tags page was finally completed, and shows every tag
with its own image.
A "see all tags" link was added in the header of every
(single) tag's page, in case of a missing description.
Also, the "secondary navigation" div now has a relative
position and a higher z-index, so that the tags page's
header does not cast a shadow on it.
Signed-off-by: Lorenzo DellacĂ <lorenzo.dellaca@mind-overflow.net>