Use Bootstrap for enhanced views

[hochleitner]

0.1.0's views are simple and unformatted HTML/PHP views. With the addition of Twig (see #1) and also for these existing views, it's desirable to achieve a decent basic look. Bootstrap comes with out-of-the-box default styles that do not require extensive self-developed CSS styles. Additionally, Bootstrap is available via Composer too, so all files remain in the vendor directory and do not pollute the views directory (which might be confusing for students).