Previously we had been showing the `name` field for the initial value of
an already selected line. However, that doesn't show all the information
that's normally given in the drop-down options. We instead have to use
the `#display_name` method to get the full display that people are used
to seeing in the drop-down.

This also means we can no longer `pluck`, as more fields are required to
assemble the display name. Instead, just do a simple `find` for the
record.

Refs #5889

After adding the `initvalue` data attribute, the placeholder was no
longer rendered. Fix this by:

1. Removing all elements from the collection to ensure that boolean
   options don't get added by default to the select
   (https://github.com/plataformatec/simple_form/blob/84e4465/lib/simple_form/inputs/collection_input.rb#L36-L41)
2. Not setting an `initSelection` on the Select2 config if the initial
   selection fields are empty.

This finally gets the placeholder to display correctly again.

Refs #5889

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