Cleanup: removed some vendor-prefixed properties

The unprefixed versions can be used in the browsers we support.
Cleanup: removed IE-specific stylesheet

Conditional comments are deprecated since IE 10.
Calculate Unix epoch using Django's own dateformat module
Use a `dateformat` template filter to display localized dates

The new `dateformat` filter allows extra formatting options and it can be
used in other scenarios too.
Simplify code, be explicit
DRY: added template tag to reference static page URLs
Cleanup: removed dead stylesheets