[WIP] Feature: add deadlines to tasks

[ms-vx]

Hi.
I've been experimenting a bit with tempo, and felt the need to add a few things to the Tasks feature.

This PR is the first step in those new features

I added a deadline date field to tasks

The deadline is displayed above the card, in red if the task is late, and red otherwise

This is not quite ready for merge yet:

•  there's a migration issue that I haven't gotten around to debug/fix: tasks created before applying the patch are acting weird.
• the way dates are compared is ugly and needs to be fixed
•  there are a few leftovers from tests I need to cleanup

I'm sending the PR in case there's interest in merging, or feedback on how I'm doing things.