Limit the number of applications a user can submit

[dirn]

Currently a user can submission an unbounded number of applications. This isn't
really of any benefit to them. We should limit the number of applications a user
can submit to one per application type. (Ideally there;d be a way to transition
an application between types, thereby allowing us to drop the limit to one
application per user.)

Before this can be done, though, users will need to be able to edit their
applications after they've been submitted. See #53.