Fix custom column width appearance combined with the Overlap columns block style

[kjellr]

As noted in #14 (comment), the flex-grow property added in #372 messes with the ability for folks to customize column width. This PR removes that and specifies a justify-content on the parent instead. The result is mostly the same.

The issue with between 600-781px that was fixed in #372 is still mostly solved. The spacing is very slightly different in that scenario (it doesn't go 100% the full width), but it still looks close to what it does in the editor, so I think it's a decent improvement for now.

Test instructions

This PR can be tested by following these steps:

1. Add a couple columns, set them to the "Overlap" block style.
2. Modify the width of one of the columns.
3. Verify that the front end matches the editor.

Screenshots

Before

Editor	Front End

After

Editor	Front End