[5.x] Retain UTM fields in listing pages

[Jade-GG]

ref: HDB-1060

Right now we end up stripping away every param from the route that's not a valid filter. However, we may want to retain the utm fields for tracking reasons.

Right now I've specifically focused on those fields, as it's harder to figure out exactly which fields are not part of the filters set. Also note that this will cause any attributes starting with utm_ to no longer work with the routing.

[indykoning]

Is there any reason we're stripping any url params to begin with? Or a mere side effect from the stateToRoute function?
Wouldn't it be simpler to merge all current url params with the information in stateToRoute?

This way we never remove params not referenced by the uiState

[Jade-GG]

Is there any reason we're stripping any url params to begin with? Or a mere side effect from the stateToRoute function?

It is a side effect; We take all parameters that don't match with known strings and put it into the refinementList in routeToState, and then InstantSearch filters out anything from the state that doesn't match an existing filter. This then gets passed back into the stateToRoute.