Infinite Scroll: allow theme to define its own click handler when using the `click` type of IS

Merges r95922-wpcom.
Infinite Scroll: ensure the theme-specified container is present before interacting with it.

When checking if additional posts should be loaded, the position of the theme-specified posts container is checked. If that container isn't present, say because no posts were found and the index template doesn't include the specified HTML ID, JS errors result that break notifications etc.

Fixes #6469.

Merges r96055-wpcom.
Infinite Scroll: customize "load more" button using custom post type's labels, when applicable

Custom post types accept an array of labels, but Infinite Scroll has always defaulted to "Older posts." With this change, IS for a single CPT query will tailor the load-more button text to use its label in place of "posts."

Fixes #6484.

Merges r96079-wpcom.
Rest API: adding timezone details to blog options

http://[private link]#comment-749

Merges r95777-wpcom.
Rest API: refine the GMT offset to a float - forgot it can be in units less than an hour

Merges r95780-wpcom.
REST API: Fix an invalid argument error. Always check that the array value is actually there before using it...

Merges r95825-wpcom.
REST API

Support /media/new upload via URL.

Merges r96056-wpcom.
REST API

media upload
temp debug to test why ifttt upload isnt working

Merges r96058-wpcom.
REST API

Some more file upload debug.

Merges r96060-wpcom.
REST API

Meida upload by URL, Switch back to sideload function. It's whats being used for the posts endpoint

Merges r96066-wpcom.
Widget Visibility: Allow the use of custom taxonomies for determining widget visibility, based on a patch submitted by a user of Jetpack. Some minor differences in my version:

* I limit term fetching to 250 items rather than 1000, which seems high.
* I limit display of the Taxonomy option so that it appears only when there are non-builtin taxonomies. This'll let it show for VIPs but not show an empty Taxonomy select box for all other wpcom users. The method I've used here is safe for Jetpack too, since it's not doing a VIP check or anything.

See https://github.com/Automattic/jetpack/issues/223
See https://plugins.trac.wordpress.org/attachment/ticket/1975/widget-conditions.php.patch
See http://[private link]

Merges r95808-wpcom.
Archives Shortcode: If the limit parses out to 0 then ignore it and use the default limit because it makes no sense.

Relates to: http://[private link]

Merges r96044-wpcom.
WPCOM: Shortcode: update from bandcamp on their shortcode.

Notes on the change from Neal Tucker ( ntucker@august20th.com ) @ Bandcamp:

We've added the ability to specify both the 'album' and 'track' parameters at once, to allow specifying a specific track to queue up on an album player.  This required a small amount of shuffling since there was an assumption that it would be either/or.  I also noticed that somehow, HTTPS handling got lost at some point, so I opted to remove the scheme from the URL entirely, leaving it as "//bandcamp.com/..."  Last time we had a conversation about that, the rationale for not using that approach was a windows XP issue, and it seems safe to ignore that now.  If you disagree, let me know and/or feel free to just change it.

Merges r95952-wpcom.