Fix import of Selectize Bootstrap theme

https://trello.com/c/dKvXkGYU/44-selectize-not-properly-working-on-staging

The problem was the .css, the actual file has .scss as extension and for some reason this didn't break in develop. But the extension is optional anyway, the asset pipeline already handles this.

I did a test deploy to staging (by changing the branch locally in config/deploy/staging.rb) and it seems to work fine now, see e.g. https://staging.aoz-freiwillige.ch/events/4. On the intro course events the dropdown is empty because Volunteer.needs_intro_course is currently empty.