Fix non-existent class navbar-top

Replaced with the proper bootstrap class navbar-static-top and also
afjusted the size of the notification toggler to match the navbar
height. Fixed some whitespace in the stylesheet too.
Toggle the user nav dropdown when clicking others

Fix the behavior of the user dropdown menu in the nav bar. When opening
one of the other dropdowns, nitpick or current exercises, close the user
menu. Also adds a grey background to the user icon indicating when the dropdown
shows.
Hide scrollbars in codeblocks when lines are small

As of 1d4d51a code displays with syntax highlighting would always have
horizontal scollbars shown, even if the code is not too wide to be
displayed wihout. Setting `overflow-x: auto;` fixes this.
Make notifications open with transition in chorme

The notification bar would not slowly slide in like in other browsers
when using chorme. Not changing the positioning form `relative` to
`absolute` when toggeling fixes this.
Remove a doubled css property
Make the notification list only cover 90% height

If you open the notification list on small pages it can get higher than
the browser window if you have many notification. This would cause
double scollbars when only one is necessary. By changing the maximum
height to 90% there appears a small grey bar at the bottom of the
notification bar, but it should fix the problem with new extra
scrollbars appearing for most screen sizes.
Merge pull request #738 from bxt/some-css-fixes

Some minor css/html fixes