Add basic schedule PDF building

Currently supports building both complete schedule and single-room
schedule (for door printouts), on both A4 and A3. Major missing feature
is figuring out a way to print across multiple pages for long days,
to fit more (e.g. two landscape A4 instead of one portrait A3, since we
typically might not have a printer being able to do A3)
Support splitting a schedule across multiple days

This will allow for example printing of two landscape A4 pages that can
be put underneath each other.

We only support this for the full schedule, not room-specific one - the room
specific ones will take so little space anyway since they are not rendered to
a relative-to-per-second size.

Splitting of pages will *only* happen on cross-schedule sessions. For each
page split we will pick the closest one to the number of pages requested and
split there. The split session (e.g. "lunch") is included on both pages, to
make it easier to follow. (We could make that configurable in the future, perhaps?)
Add some help texts to form output, and some css styling
Don't crash on tracks with no color
For tracks with no color, enforce white background

Otherwise we can end up with black on black at the beginning of
a page..
Typo fix
Add ability to filter by track in pdf schedules

This makes it possible to for example exclude parties from the printed
schedule. All tracks are included by default.
Include room names in the full day schedules

Makes it a lot more useful :)
Include speaker names in full schedule as well