[core] fix dialog layout regressions, use DialogBody more

[adidahiya]

Changes proposed in this pull request:

This fixes some layout regressions that were introduced with #5753.

Fix 1:

First, I've brought back the dialog body margin on <div className={Classes.DIALOG_BODY}> elements. We need this style to preserve backcompat of our CSS API:

However, <DialogBody> can continue to use the (better) padding style instead of margin. This is ok because it's a new component introduced in core v4.14.0. This means that overflow scrolling still looks good:

Fix 2:

The new MultistepDialog styles broke when there was no left section / no close button. This was due to the removal of a container element in the footer refactor in #5753. I've brought back that container element even in the case where there is no close button, and also renamed it to "main-section" instead of "left-section".

before	after

Internal enhancement:

I've migrated the rest of this monorepo to use <DialogBody> more consistently so that we can better dogfood it and prevent regressions like this.

[adidahiya]

[core] fix dialog layout regressions, use DialogBody in more places

Build artifact links for this commit: documentation | landing | table | demo

This is an automated comment from the deploy-preview CircleCI job.

[adidahiya]

fix testS

Build artifact links for this commit: documentation | landing | table | demo

This is an automated comment from the deploy-preview CircleCI job.

[adidahiya]

footer-left-section -> footer-main-section

Build artifact links for this commit: documentation | landing | table | demo

This is an automated comment from the deploy-preview CircleCI job.