feat(media): add max size mode

[oOOoutsiderOOo]

Added a new option to set a fixed width for the media widget. Using size_mode: "auto" it behaves as normal, following the label width. When setting size_mode: "max", it renders at max_field_size width regardless of the actual label width.

[Video-Nomad]

Thanks for the PR!

Is there a way to make this work with stylesheet alone?
This will set the fixed width of the label, but now that I test it it does not behave reliably, so I get where this PR comes from.

.media-widget .label {
    min-width: 400px;
    max-width: 400px;
}

Would be great if we could control that via stylesheet alone.