Fix responsive column span logic on the front end.

[tellthemachines]

What?

Fixes issue detected here where grid elements spanning multiple columns weren't correctly downsizing when the parent grid's minimumColumnWidth was a non-default value.

Testing Instructions

1. Add a Grid block to a post or template, and add some children to it.
2. Adjust the column span values on some of the children so they cover multiple columns.
3. Save, publish and view on the front end. Resize browser window and check that multi-column elements reflow nicely.

Testing Instructions for Keyboard

Screenshots or screencast

This should be the expected behaviour for the example given in the original report:

grid-resize.mp4

[github-actions]

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Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

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[fabiankaegy]

This works as expected :)

[tellthemachines]

Adding backport label so this fix can hopefully go into 18.2! Cc @colorful-tones

[colorful-tones]

Thanks for the heads up @tellthemachines. I'll do my best and follow the notes to cherry-pick this in, and of course, ask questions if I hit a snag. 👍

[colorful-tones]

I just cherry-picked this PR to the release/18.2 branch to get it included in the next release: fd37383