Frontend/Rework: Welcome page

[TimKardol]

The welcome page needs to be updated with at least the new date as well as some design decisions.

• [ ] Set new conference date for countdown
• [ ] Use new pictures for the carousel
• [ ] Restructure pages using components (?)
• [ ] Default message when no sponsor have applied
• [ ] Fix styling of applied sponsors

[TimKardol]

Closed in #401