upgrades to invitations

[doomlab]

• How to invite more than one contributor at a time (multiple email?)
• When inviting, the conformation button should be “send invite” not “add contributor” and use different colour for the button (e.g. green)
• After inviting the screen goes to dashboard - would be better if it went back to contributors
• Allow to customise invitation text - e.g. by having a box for entering a custom message.
• “Invited Contributors” page should show invitation status (whether it has been accepted)