TransmissionId should only be allowed on activity type TransmissionOpened

[oskogstad]

Description

The validator activity validator allows setting TransmissionId on every activity type except DialogOpened.
It should only allow on TransmissionOpened.

Reproduction

Create an activity, f.ex. of type DialogCreated, and set transmissionId to a valid UUID (ie, a transmission on the dialog)

Expected behavior

400 BAD REQUEST saying transmissionId is not allowed for this activity type

Actual behavior

201 CREATED