base: sticky footer fix

* Fixes sticky footer issue on pages with content longer than
  100% window size, by updating to the reference sticky footer
  implementation for Bootstrap3.

* Uses LESS `@import (reference)` to import external files, but
  without adding the imported styles to the compiled output
  unless referenced.

Signed-off-by: Georgios Kokosioulis <georgios.kokosioulis@cern.ch>
base: visual fix of the user's tab on the header

Signed-off-by: Georgios Kokosioulis <georgios.kokosioulis@cern.ch>