admin: Add timeline widget

[katajakasa]

Add a timeline widget to the event dashboard that shows a timeline with stuff that has passed and stuff that is upcoming.

• Horizontally scrolling events (from right to left)
• A vertical line in the middle, a line that snows the "now". On the right, upcoming stuff and on the left, passed stuff.
• Different event types in different colors
• Clicking an event takes one to view that specific thing (?)