Fix Screwdriver theme

Before, it wasn't possible to hide categories when they had no articles to read.
I applied the same rule than the one described in here (https://github.com/marienfressinaud/FreshRSS/commit/d19824b919289ad63743f27da7861f2422da5baa).
Now, the categories are hidden when they have no articles to read.

See #594
Merge pull request #598 from aledeg/screwdriver

Fix Screwdriver theme