Internal: Small improvements to SortableTable logic, Users page and Colorized courses table

SortableTable

• The sorting method has been made a lot more simple and it's now able to handle Date objects as well

Users

• is_enabled field was removed, this was only needed when the old user service was used, now it was always set to true
• Looking for users based on their roles, programme rights or IAM groups is now easier, thanks to the multi-value filter

Colorized courses table

• Calculating the stats is now faster in the backend: certain aggregate values weren't needed as they are calculated in the frontend