Flesh out pre-conference deadlines

We now have the timeline for the conference itself mapped out: https://debconf19.debconf.org/schedule/important-dates/

What's missing is the timeline leading up to it. Compare to e.g. https://debconf18.debconf.org/schedule/important-dates/

I propose:

1. Closing registration for accommodation, t-shirts, etc. 1 month before the start of DebCamp, again.
2. Closing talk submissions 2 months before the start of DebCamp.
3. Opening registration early in the new year. Late Jan?
4. Closing bursary applications 1 month after registration opens.