Media Summary: Setup postdata for obtaining the excerpt

[kraftbj]

Fixes #8420
Fixes #9209
Complement of #8510
Supports #8156

In PHP 7.2, warnings are thrown, in the end, because we are attempting to apply get_the_excerpt filters without being within the Loop. While Core makes it seem like this should work, it doesn't. (see #8510 for PR that would require Core changes to work as I think it should).

In this PR, we override the $post global and setup the post data so the excerpt and underlying functions work as intended.

We're overriding the $post since this function does not presume to operate within the Loop, so we need to fully mock the Loop.

Testing instructions:

• Jetpack + Sharing (or any module that outputs OG tags) on PHP 7.2
• Open a post without an explicitly set excerpt.
• Observe PHP warnings are reported in PHP 7.2: Warning: count(): Parameter must be an array or an object that implements Countable in...  #8420.
• Apply patch.
• Confirm excerpt is set in the og tags in head with no PHP warnings.

[oskosk]

LGTM!

[oskosk]

@kraftbj should we leave an inline note (/todo) specifying that this code could be removed if core fixes something ?

[kraftbj]

@oskosk Good idea. Added via 467eb0e .