Split up _find_review_request to retrieve the ReviewRequest object, then check permissions.

Currently, `_find_review_request` retrieves the ReviewRequest object matching an
ID and checks permissions, returning `None` if the review request is not
accessible by the user. Splitting this up into two parts (retrieve the
ReviewRequest object first, then check for permissions) allows us to get a
ReviewRequest object in cases where permissions are not needed.

Testing Done:
- Manually checked that the review request details page shows up.
- Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5603/