Warn users before they request to join a team that’s already partway through the process

[zarino]

Broken out from #14:

One additional feature that I could create a ticket for is something like a dialogue box so that if users try to join a team doing anything other than an assessment, they're served a message saying "Here's why we think you should do an assessment..." with a button or two to let them carry on or return to search results. — @sequencefree

Depends on #15.