Implement navigation to sub-routes.

[Sintrastes]

Currently, our Page type is just an enum.

However, in order to support features such as #19, we will need to add "sub-routes" (such as "anchors" to specific types/entities) into this data type.

This should be fairly straightforward to do with Haskell's ADTs. Example:

data Route =
    Home
    ...
  | Entities { anchor :: Maybe EntityId }
    ...

As the application becomes more complex, this would also allow for things like specific preference pages.