Fix API 422 error: `Duplicate value for "description"`

When looking up an existing hub token among user's authorizations, be
sure to fetch all results by following pagination links. Previously we
just stopped at the 1st page, and if the token for hub didn't appear
there we would try to create it, which would result in a 422 validation
error if it already existed.

References 1e4477b566c01aadfd807fc77ab4596ec164aed2

Fixes #531, fixes #558, fixes #563
Merge branch 'auth-pagination'