Added invitation buttons both in members index and view

closes #90 (closed)

Most of the stuff described in #90 (closed) changed since then, here's what we finally have done:

• Show an invite link in the members index (in the validated column)
• Show an invite link in the members view
• When pressing the link a confirm box is shown
  • If success
    • Show a success modal
  • else
    • Show error with an error modal