Day headers on the left

[yvolk]

This is an old suggestion by @easycheese
They even provided a PR for that (it was outdated unfortunately, when I looked at it...) plusonelabs#267

Interesting layout.
I would call it "Day headers on the left" option (in addition to show/hide Day header, which we have now) so we could have three options for Date headers:

Above events
To the left of events
Hidden.