Use templating to render homepage

Rather than FileSystem.Open
Render html template in Executor.
Rename homepage.html to homepage.html.tmpl
Refactor to split Executor methods across separate files.
GET to /light returns 503 if light state unknown.
Don't render light controls on homepage if light state unknown.
Return 503 at /webcam when error occurred.
GET to /light returns json representation of state.
Use consistent 'Should' language in tests.