Change captions to use <figure> and <figcaption>

[kinglozzer]

Notes:

• Hitting return when editing a <figcaption> will result in a new <figcaption> being created, rather than a new paragraph outside of the <figure>. The same issue already exists in v4 - editing a caption and hitting return will add another <p class="caption"> instead of breaking out of the containing div. On save, they both get concatenated into a single caption though.
• The embed with caption is a <figure> in TinyMCE, but not when the shortcode is rendered. That’ll need a separate PR to framework to update the shortcode template
• I wasn’t sure whether the changes to extended_valid_elements should be made here or in framework
• I also wasn’t sure if an embed without a caption should still be wrapped in a <figure> instead of a <div> like it currently is. I kept it as a <div>, but it might be better to just always use a <figure> so it’s a predictable element. Apparently a figure without a figcaption is fine.