Add explicit box-sizing to crm-flex-box style

[colemanw]

Overview

This ensures consistent box sizing when using the new crm-flex-box css class, for consistency with or without Bootstrap 3.

Technical Details

Bootstrap 3 sets this box-sizing value explicitly. We should too.

[civibot]

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[eileenmcnaughton]

@colemanw I'm inclined to merge this before the rc is cut due it presumably playing into other theming changes. However, this isn't my area - is there someone who is likely to weigh in on this or some other reason I should feel comfortable merging?

[colemanw]

Well, it doesn't currently make any difference. It's more like future-proofing so we don't end up with theming ambiguity.
I checked the dashboard page and it looks exactly the same before/after.

[seamuslee001]

This seems ok to me merging