Allow section decoupling

As an API user I want to be able to decouple section from transect and leave it in unassigned state.

AC

• It is possible to set transect to null in update section mutation
• Only admins and project leads can decouple section from transect (set transect to null in section update)
• Section cannot be created without transect
• Check possible data integrity issues when section is decoupled (existing section events, observations, etc.)
• Tests

Review

• It is possible to set transect to null in update section mutation
• Only admins and project leads can decouple section from transect (set transect to null in section update)
• Section cannot be created without transect
• Check possible data integrity issues when section is decoupled (existing section events, observations, etc.)
• Tests