fix export button and file extension
add event url
Various things, please see description.

- Removed the social-share gem, as it didn’t load correctly most of the
time and it only had Facebook like functionality instead of share.
- Added a js social share plugin instead.
- Got rid of some unnecessary js files .
- Had to get rid of turbolinks as it stopped some other JS files to
load correctly without having to refresh page.
- Changed text, size and position of Calendar export button.
- The correct URL of an event is now also in the calendar file
Merge branch 'feature/fix_export_ical' into dev