Step 1: Send invitation

See tickets 1838 and 1848

Step 2: User receives email

See ticket 1902 

Email should be limited to 2 links, one to Openplans in general for more information about it and
one to accept the invitation and learn more.

Step 3: Landing page from "accept this invitation" link

Step 4: Landing page once user has joined

Reverse order of things but keep 1 page tour appraoch

Main body should contain:

* Go to project that invited you
* See other members of this project (maybe?)
* See profile to person that invited you
* Update your own profile

Side bar should contain:

* Explore more projects
* Start your own project
* Learn more about this site