Split the invitation creation into two screens

[mikron-ia]

Currently, it is not possible to see the invitational e-mail as it will appear. Add a preview of the full content. The edit page would create the invitation, but explicitly not send it yet.

On the form page:

• change the button to Prepare invitation

On the preview page:

• this does not have to contain the link (it is not generated at this stage), there can be a placeholder.
• display the subject and the content separately
• add previous consent needed warning
• consider adding From: field display as well (the e-mail from which it will be send)
• use the proper Send button

On the index page:

• Add view option and move all other actions to the view screen