Check if the user wants to receive mobile notifications

[enricostano]

A push notification can be created only if:

• the user that would receive it has push_notifications set to true
• the user is an active member of the organization

Related to #356

[sauloperez]

What is this exactly? We want forcefully ask all current users? Just wondering

[enricostano]

We just need to filter out the people that set the push_notifications setting to false while creating PushNotification.