Add option for assign as observer default value

[RomainLvr]

The customer requests that the “Assign me as observer” box be unchecked by default.

Related ticket : 33253

[Rom1-B]

We're very likely to get the opposite demand: people who want it checked by default.
In this case, we'll need a new parameter.

[RomainLvr]

We're very likely to get the opposite demand: people who want it checked by default. In this case, we'll need a new parameter.

Sorry for the mistake, it's all done 1cdbadd 👍