Enable support for side-by-side mode for all web pages, behind an
`html_side_by_side` feature flag.

 - Add `html_side_by_side` flag to known flags in `features.js`

 - Define `FeatureFlags` interface for querying (but not updating flags)
   and make the `FeatureFlags` class implement it

 - Pass the `FeatureFlags` instance from Guest through to
   HTMLIntegration and read/observe the `html_side_by_side` flag to
   enable/disable side-by-side mode

 - Create a dummy `FeatureFlags` implementation in
   `VitalSourceContentIntegration` which always has the
   `html_side_by_side` flag turned on