Add ability to re-generate schemas as required.

[Decipher]

As a developer I don't want to restart my frontend every time the backend display modes are updated.

Currently the schema is only generated on first run/build. This means that if the Display modes are updated in the backend while the Frontend server is running the schema will not reflect the active backend configuration.

Due to the current permissions required for druxt-schema, this will either require authentication, restricting usage to developers, or a Read-only permission in the backend.