HG-328 Add :active pseudo class to login anchor in site head

There was an inconsistency in site header in Mercury when you tap navigation bar button and hold it its background changed color to gray. But when you did the same to user login button it didn't indicate "hovering" in the same way.

Changes in this pull request make the state for both buttons in site header consistent.

Btw. I've added `test/fixtures` directory to `.gitignore` :japanese_ogre: 
it'd be best if this was a separate PR,
but it's okey now