The Nimbus Features corresponding to FxMS messaging surfaces are actually
intended to map to FxMS message groups, which can accept *any* FxMS message.
The features have been updated with schemas that accept any FxMS message.
As part of this, all FxMS schemas have been updated with an `$id` so that they
can be bundled into feature schemas and have their internal `$ref`s work.
(Otherwise, a `$ref` would be relative to the top-level schema instead of the
sub-schema).
Schemas for individual message types are no longer exposed as resource:// URIs,
except in tests, as indivual schemas are no longer required at runtime.
Additionally, each FxMS schema has had its `template` field become required and
requires a constant value for that schema (e.g., Spotlight requires a template
value of "spotlight").
A test has been added to ensure that if any of the messaging surfaces schemas
change that the feature schemas are also updated. The feature schemas can be
regenerated via:
```
cd ./browser/components/newtab/content-src/asrouter/schemas
../../../../../../mach make-schemas.py
```
Differential Revision: https://phabricator.services.mozilla.com/D147332
b3e2605f2b