Added /invites backend apis in frontend. See #25, see #27,
Added a basic invitation page. See #27. Can display and send invitation.

Right now we have the abililty to display existing invitations and send
new invitation (and reload the page to show the addition). We still need
to hook up resendInvite and removeInvite and a filter. Backend does not
have DELETE method yet, so we will implement that later.
Fixed #27. client-side filter and sorting should be working.
Undo 54957de. Invite belongs to invitation page. Close #25.