Enable admins to send bulk invitations to participants

A critical point for administrations when launching a participatory process (or a Decidim instance altogether) is to acquire participants. It requires an extra effort for the communication (financial and otherwise) from the administration. It could be eased with the ability to invite willing participants in bulk. Prospects and clients regularly ask us this feature. Obviously the main obstacle to such a feature is GDPR and collecting the consent pre-invitation. To address it, we could emulate what is done by email delivery services when importing contacts for a campaign. You have to certify that you are following GDPR rules. We attached a screenshot of what Mailjet does. We could also add some other checks. For instance your import file must include the date you got the consent for each user, what was used to collect the consent (paper, url from online surveys, etc.). Include an opt-out to the email invitation and so on, so that inviting participant isn't too straightforward.