Move feature flag initialization logic out of lib.rs. (#11380)

## Description

Trying to do a little codebase clean-up. We've got way too much stuff in
`app/src/lib.rs`, which should probably be mostly (if not only?) module
definitions, plus perhaps a couple public re-exports (for integration
tests and such).

We already have a `warp::features` module (which is where the feature
flags used ot be) - seems like a good place for `init_feature_flags()`.

Author: vorporeal