Modal header/footer no border is not always applied

Problem/Motivation

When unchecking modal option 'Insert horizontal line' for either modal header or footer, the modal-no-border class is applied to the respective container that removes the border.

This works well when modal_page module's css is attached after bootstrap css, however, in our use case, we are using a theme that is building the bootstrap css and attaches that stylesheet after modal_page's css (loading Bootstrap via cdn is disabled, as the theme provides Bootstrap assets).

This could be fixed if the border: none !important rule would be used, so that the border is removed regardless when the css is attached.

Steps to reproduce

As written above, to reproduce, you need a Bootstrap based theme that loads the assets after the module's css is loaded. In that case Bootstrap's rule that defines the border on the header/footer will take precedence over modal_page's rule.

I'm not 100% sure if this qualifies as a bug, but IMO the styling should make sure in this case to always remove the border. Feel free to change the issue to improvement if you don't agree.

Proposed resolution

The ideal would be to use Bootstrap's border utility class border-0 here, but unfortunately that is not available in Bootstrap 3.

So in order to keep supporting Bootstrap 3, we could add !important to the custom class.

Remaining tasks

Add !important to the border removing rule for class .modal-no-border.

User interface changes

API changes

Data model changes