docs.devilry_qualifiesforexam: Matching button styles.
devilry_subjectadmin...ListOfPeriods: Replaced timespan class.

It collides with the ``*span`` bootstrap css selector.
devilry_theme: Move forms and layouts styles into the bootstrapextras class.
devilry_theme: Match button styles for ExtJS to the bootstrap look.

We tuned the ExtJS styles, and they are now quite similar, except for
the "small" style which is sligtly taller in the bootstrap style.