Handle API endpoints with or without trailing slashes, without redirecting.

Summary:
Note that by applying this globally, we're being permissive not just
for, say, `/threads/` vs `/threads`, but also for
`/threads/ckjjbxbxda48hr07fvqu644pj` vs `/threads/ckjjbxbxda48hr07fvqu644pj/`.
If this isn't intended behavior, we should instead set strict_slashes=False
only on specific endpoints.

Test Plan: Do `curl http://localhost:5555/n` and `curl http://localhost:5555/n/`.

Reviewers: mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D191