Router tweaks

[japgolly]

From MainRouter in ochrons/scalajs-spa-tutorial.

I noticed two things:
BaseUrl is initalised with a custom function instead of one of the helpers on BaseUrl. Are the built-in helpers not enough? (Why?) If so, should we move that takeWhile function into BaseUrl as a new helper?
val routerEngine = ... ; val router = .... There are already defs in RouterRules that do the same thing. > Do you I think I should just change them to vals?

the defs create a new router every time they are called, so that's why I put it into a val
routerComponent is realistically used only once, so it doesn't really matter, it could be def as well
the BaseUrl stuff is just to support the client-only version, where the URL contains a long path into the "filesystem" due to the way the workspace plugin works

[japgolly]

Resolved via 93bbc5f