[17.0][FIX]``mail_activity_team``: do not systematically remove the user from an activity when a team is set

[Camille0907]

In this PR, we updated the module to follow the v17 changes in standard. However it looks like the changes made are too wide: if no user is set, we indeed want to prevent the standard method to set one by default. But if a user is manually added, we want to keep it. Here we add a context key to determine at the creation of the activity if the user should be kept or erased. We also take this issue into account.