Services in `src/sidebar/services` are effectively classes. They are
currently implemented with a convention that was more common pre-ES6 where
the constructor is a function that creates the fields as local variables
and the methods using closures and returns an object that references the
closures.

This pattern has advantages, especially pre-ES6, as it avoids issues with
incorrect use of `this` and hides internal state from consumers. However
it also has downsides:

 - It is less obvious to readers that they are looking at something that
   is logically a class

 - This is not an idiom we use elsewhere in the codebase, where we use
   native classes instead

 - Static analysis tools don't support this pattern for creating a class
   as well as they support a native class. For example `TS` creates a
   named type for native classes, which is convenient to reference
   in JSDoc comments

This commit starts a process of refactoring service classes to ES
classes which are named `<Thing>Service`, using the router service as a
first example. Per recently agreed conventions, the classes are named
rather than default exports.