Implement Manchester requirements

See merge request !26

This reverts commit f15b075e.

This reverts commit 52352ddf.

This reverts commit a7622b5d.

This reverts commit 68ba7dbf.

This reverts commit 6990ae44.

This reverts commit 354f502c.

This reverts commit d948ff2a.

This reverts commit 745a0b07.

This reverts commit ff28c230.

This reverts commit 976fa45e.

This reverts commit e2cc32b6.

This reverts commit 9cbc3e08.

This reverts commit b861bf2b.

This reverts commit 89323c5b.

- removes
  - Poster
  - Demonstration
  - Keynote
  - Lunch
  - Party
- changes
  - Fika to Tea Break
  - Book/Special Issue to Book

#261: added mandatory conference properties to seed

See merge request !25

Fix #199, fix migrations

Closes #199

See merge request !23

attempt correcting display of overlapping events

Closes #246 and #243

See merge request degrowth/scriptorium!22