Revised dialog model

[elsand]

This is the meta-issue tracking the work for implementing the new dialog model. Current written draft is available at https://gist.github.com/elsand/ef205ad479b3377aea98506808d9a84f

This list is work-in-progress

## Tracked issues
- [ ] https://github.com/digdir/dialogporten/issues/856
- [ ] https://github.com/digdir/dialogporten/issues/857
- [ ] https://github.com/digdir/dialogporten/issues/851
- [ ] https://github.com/digdir/dialogporten/issues/860
- [ ] https://github.com/digdir/dialogporten/issues/858
- [ ] https://github.com/digdir/dialogporten/issues/859
- [ ] #752
- [x] Remove PUT/DELETE endpoints for dialogelements (now transmissions)