Allow the hero to be filtered

[wjhdev]

These Changes abstracts the featured media stuff that Ryan built to make it possible to filter the generated hero DOM for overriding in child themes.

Changes:

• partials/single-hero.php is removed and replaced with two functions: largo_hero() (prints) and largo_get_hero() (returns).
• Removes error checking where error checking is already done.
• Replaces largo_hero_with_caption() with largo_featured_image_hero(). This is to keep consistency with newly introduced largo_featured_gallery_hero(), largo_featured_embed_hero(), &c. naming convention. This group of functions generate slightly different DOM for their respective hero type. Suggestions on how to handle depreciation appreciated. The function still exists to keep backwards compatibility, but we should have a system for depreciation (like WordPress).
• adds largo_get_hero filter for overriding what is displayed by child themes.

[wjhdev]

This branch started from master with develop merged in, so Adam's 4311e9d commit made it in here.

[wjhdev]

Tests appear to be working now. The change in 5ab45b2 is to the actual test itself, which appears to have an erroneous assertEqual that was unreachable before.

I scratched my head a lot looking at these tests. I can't find anything documenting what WPAjaxDieContinueException is or why it gets fired. @rnagle, it might be a good idea to check that what I did is correct there.

[rnagle]

Thanks @willhaynes. I went ahead stubbed out tests for all of the new functions you added and factored the markup generated by largo_get_featured_embed_hero, largo_get_featured_image_hero and largo_get_featured_gallery_hero out into template partials for added flexibility. These changes have no practical effect on what you've done here, just another layer of flexibility and organization. Thanks for digging into this.