Allow the embedder frame to specify a list of feature flags that should be
overridden in addition to those enabled in the H user profile. This will be used
in the Hypothesis LMS app to enable feature flags to be turned on at the level
of an LMS app installation, rather than for individual users.

The ability to override flags has been limited to embedders (in practice, our
LMS app) to try and avoid a situation where arbitrary web pages enable, and
become dependent upon, feature flags that we later remove.

The flags coming from the embedder add to, rather than replacing, the profile
flags. This has been done to avoid a confusing situation where a feature flag
that is enabled for "everyone" in H actually ends up not being enabled for all
users due to eg. the LMS app disabling it for certain installations.

Part of https://github.com/hypothesis/client/issues/5803

 - Change `ConfigFromHost` type to only include settings that can come from
   either the host frame or embedder frame. Settings that can only come from
   a specific context have been moved into the `ConfigFromAnnotator` or
   `ConfigFromEmbedder` types respectively.

 - Add `features` config to `ConfigFromEmbedder`

 - Add `features` store method which combines feature flags from profile and
   global settings

 - Replace some references to `Config*` types in `build-settings-test.js`. The
   type references were wrong because RPC calls return the `ConfigFromEmbedder`
   type, not `ConfigFromHost`.