Make modal display plugin use iframe

I started working on this. Approached it like that:
- created a new controller for the modal page that returns the open modal ajax response
- changed the display annotation so we can specify which controller the display uses. A temporary solution until #2364193 is done
- use the display controller when creating routes

Using this approach I still need to figure out how to display the actual content in the iframe, because the modal route now uses the modal controller which returns commands not content.

An initial stab at this. Still needs cleanup, some more testing and more work: https://github.com/drupal-media/entity_browser/pull/111

Just want to make sure that the direction is ok.

I've yet to fix:

• implement the "only-content" theme for the page in the html theme
• figure out why the Modal object properties are null on if I press the open modal button for a second time

I've fixed the stuff I mentioned in my previous comment and I will open follow ups when this gets in.

I didn't find a way to use $form_state->get('path') in the openModal method, so I left it as it is. I thought that because it's us, who set the path element value, there shouldn't be any security concerns getting the value from input. Although admittedly not as pretty as it would be by doing that via get.

Follow ups:

• #2609308: Evaluate if we can move more code from DisplayBase extenders to DisplayBase
• #2609336: Create a command to close the modal
• #2609418: Switch the modal trigger from button to link