auto schedule release

[johanvdw]

Pretalx has no auto release currently. If changes are made to the schedule, an actual release must be made before this is active.

We need automated releases.

My proposal would be:

• Create an auto release tool: if a warning/error occurs it should not release. It should also not release if no relevant changes are made.
• Release name should be year-month-day h:m
• For starters run this tool manually at least every day
• Add it to the website build if we are happy about it.

At the end of january, we should have a view similar to https://pretalx-test.fosdem.org/democon/schedule/changelog/ (this is not visible for the non-test instance currently).

[johanvdw]

The commit above implements an auto-release:

• if there are no conflicts make a release
• send mails to speakers
• it currently just stops if there are conflicts, perhaps it should send a message somewhere

[johanvdw]

I have changed the website export so it can also read the unreleased schedule. This make releasing optional (but useful for mails to speakers or the changelog).