These are just instances that jumped out at me while working on the
subdomains code, mostly while grepping for get_subdomain call sites.
I haven't attempted a comprehensive search, and there are likely
still others left.
Now that the old `check_subdomain` has no callers except in
implementing the new, improved interface `user_matches_subdomain`,
inline it into that. Also simplify the Boolean logic a bit.
Now that every call site of check_subdomain produces its second
argument in exactly the same way, push that shared bit of logic
into a new wrapper for check_subdomain.
Also give that new function a name that says more specifically what
it's checking -- which I think is easier to articulate for this
interface than for that of check_subdomain.