Add/Edit invitation

NOTE: Do not change the link to the survey. The survey might be unreachable if it is changed. The [ID] and [KEY] is used to track the invitees, to indicate who has responded, and who has not. [REOPEN] is only used when an invitee already has completed the survey and the link has been resent from the Opinio admin module. This is another key generated from the respondent id and invitee key. When this reopen key is valid the multiple restrictions (if any) will be reset, and the invitee will be able to continue with the reopened survey. It is not used instead of, but in addition to the invitee key.