The preview deployment limit was checked before looking up whether the
pull request already had a preview, so once an application reached the
limit every `synchronize` event was dropped and existing previews went
stale while GitHub still saw a 200. The check now runs inside the branch
that creates a preview, uses the schema default of 3 when no limit is
configured (instead of 0, which blocked every app without an explicit
value) and compares with `>=` so the configured limit is inclusive.
Closes#4074
