Revert "Remove unreachable code"

This reverts commit 39615669c44a52bf54c4c411c7aeeca21144955b.
streamer: Reenable annotation event streaming
features: Add a simple feature flag harness

This commit adds a relatively simple feature flag harness which allows
us to query the activation or otherwise of named features when handling
requests.
streamer: Put streamer behind a feature flag

Only enable the "realtime" features if the "streamer" feature flag is
enabled.
Merge pull request #1686 from hypothesis/realtime-annotation

Realtime (behind a feature flag)