`.popover-body` should include only user contents

In the current configuration, we add the <button> element inside the .popover-body > div container.

<div class="popover-body">
	<div style="display:contents;">
		<!-- user content here -->
		<button type="button" aria-label="Close" class="btn-close" style="position:absolute; top:0.6rem; right:0.25rem; width:0.55rem; height:0.55rem; background-size:0.55rem;"></button>
	</div>
</div>

It'd be helpful to keep the .popover-body to just having user-supplied content. The close button could be a sibling of .popover-body without changing appearance or (probably) behavior. The goal is that users should be able to write rules like .popover-body > div > :last-child to target the final (or first) content element in the popover.

In particular, I wanted to do something like

.popover-body > div > :last-child {
  margin-bottom: 0
}

Start by tracing the popover markup that produces .popover-body > div and the positioned close button. Move toward markup where the body contains only user-supplied content while preserving the close button's appearance and behavior, then verify that selectors such as .popover-body > div > :last-child target the user's final content element.