ReferentialVehicleJourneys: Show selected line in drop-down

Previously we weren't showing the currently-selected line filter in the
drop-down label.

If a line is selected, get its ID and name and use those to pre-fill the
select.

Using Select2's `initSelection` option to configure this instead of
putting `selected: ` in the Rails field definition. That would have
looked like this:

    collection: [[@filtered_line.name, @filtered_line.id]],
    selected: params[:q] && params[:q][:route_line_id_eq],

But doing it that way wasn't possible because this is an AJAX-populated
Select2.

Since we already have a way to use `initSelection`, this was the
quickest and easiest way to go, but apparently Select2 v4.0 deprecated
this option, instead doing it with a data adapter. Couldn't really be
bothered right now to figure out how to set that up as it seemed like a
lot of work for not much payoff right now. Also, I was a little
disappointed in Select2's docs regarding this. From what I understood,
it looks like you basically need to create your own custom data adapter
to do this, which sounds like a pain, and I'm not sure if I would have
to mess with the rest of our existing Select2 configuration setup in
order to integrate that.

Refs #5889

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