Add event timeline with introduction events

[davidkrantz]

This PR replaces the introduction matrix image in the introduction tab with a timeline with the introduction events. For this to work, PR "Add API endpoint for introduction matrix" is needed. These two PR:s together closes #190.

Some information about this PR:

• Each introduction week has its own F7 vertical timeline, meaning that the events are grouped by week
• Events occurring on the same day are grouped to one side of the timeline. When an event occurring on a new day is found, it is placed on the opposite side of the timeline
• The title, location and the user's registration status are shown for each event
• The background color of the event containers changes depending on which week it belongs to and how far into that week the event is (thanks Fredrik for helping me with the CSS!)
• When the events have been loaded and the background gradient has been set, the user is automatically scrolled down to today's date
• The timeline itself is green if the event is in the past or if it is happening right now and light gray otherwise

[davidkrantz]

Looks good! I noticed that if you go to an event through the timeline, sign up and then go back the status text is not updated. It still says: "you're not signed up". Not very important and I don't know how easy of a fix this is so if you want to you can take a look at it!

Very cool changes!!!

Yes, it would be nice if the registration status could be updated. The event cards on the home tab also have this problem and I struggled with it when implementing them as well. I yet have not found a clean solution for this, but I will add an issue and get back to it later, see #204.