Unify CSS and fix weirdness with missing margins

[iTitus]

Examples:

Not Bootstrap-like, missing space around checkbox.

Bootstrap-like, but there is no margin around the +-buttons and the checkboxes' highlighting is cut off on the left.

[dpkass]

Pages missing bootstrap:

• [x] /question-set/:id
• [x] /question/:id
• [x] /create-course
• [x] /create-course-iteration
• [ ] /new-correction

Pages poorly styled:

• [x] /create-question

[iTitus]

/question/:id fixed in #178 /create-question fixed in #179 /create-course fixed in #182 /create-course-iteration fixed in #189 /question-set/:id fixed in #190