Add SPA support

CHANGELOG.rst

aleksis/apps/alsijil/frontend/index.js

aleksis/apps/alsijil/frontend/messages/de.json

aleksis/apps/alsijil/frontend/messages/en.json

aleksis/apps/alsijil/static/css/alsijil/lesson.css

...apps/alsijil/templates/alsijil/class_register/lesson.html

...ps/alsijil/templates/alsijil/partials/lesson/heading.html

...templates/alsijil/partials/lesson/tabs/documentation.html

.../alsijil/templates/alsijil/partials/lesson/tabs/more.html

...alsijil/templates/alsijil/partials/lesson/tabs/notes.html

aleksis/apps/alsijil/views.py