This reverts commit f6787df785c8c8e2bdbf4d7b65a9176adad64d19, reversing
changes made to 29d975367171b8d4e69941c3f0e5d40ddfc98ddc.

I'm afraid I've had to revert this:

- it introduced a crashing bug on master (see
  https://github.com/hypothesis/h/pull/2131#discussion_r30692024)
- feature flags are no longer accessible by view functions (the
  configurator object is not available from the request), thus this
  blocks #2131