Improve invitation accept process

[gempain]

See background in #177 and #223 and #226.

When you click an invite link, the UI should ask you to create an account or login while being aware that you're in the process of accepting an invite. We could show a message saying "You need to be logged in to view an invite".

We get a 500 when accepting an invite for an org we're already a member of. Seems like the invite is being removed as refreshing the page gives a 404.