Infinite Scroll: use sane priority for `wp_footer` output

Unnecessarily-high priority is unnecessarily high. We only need to ensure our callback fires after Core prints its footer scripts.

See #6588.

Merges r97389-wpcom.
Infinite Scroll: accomodate Core's media player

Before initializing Core's implementation of the 'MediaElement.js', ensure the library itself is loaded. Otherwise, errors result and the media player isn't shown.

Fixes #6588.

Merges r97395-wpcom.
Infinite Scroll: accomodate Core's media player

Before initializing Core's implementation of the 'MediaElement.js', ensure the library itself is loaded. Otherwise, errors result and the media player isn't shown.

Fixes #6588.

Merges r97395-wpcom.
Infinite Scroll: bind `setTimeout` handler properly so we don't need to reference the singleton.

Thanks to @miguelcsf's excellent explanation of 'this' in the context of `setTimeout`, I don't need to reference the IS object directly via the global conext. Huzzah!

See #6588.

Merges r97398-wpcom.
Infinite Scroll: ensure Core's media players are initialized with each IS load, but only initialized once

r97395 added special handling for Core's ''MediaElement.js''-based players needed by incoming post sets. That changeset, however, failed to account for the situation where players are present in the initial set of posts and in subsequent sets of posts loaded by IS. This changeset ensures that all players, regardless of when they're added, are initialized.

''MediaElement.js'' doesn't like it when already-initialized players are re-initialized, which complicated this changeset somewhat. Re-initializing initialized players causes them to break completely, which is why we unbind and rebind the custom initialization.

This changeset also causes IS to pass the response object when triggering its `post-load` event, as suggested by @bradyvercher in https://github.com/Automattic/jetpack/pull/532. Doing so allows the Mejs initialization to bail early when there are no incoming players to initialize.

Fixes #6588.

Merges r97399-wpcom.
Infinite Scroll: remove superfluous whitespace introduced in r97399.

See #6588.

Merges r97400-wpcom.