Tiled Gallery: Fix image meta encoding.

Image meta is now encoded by calling `esc_attr` on the result of mapping strval over the image meta attributes, instead of mapping `esc_attr`. This way `json_encode` can properly escape any quotes inside the attribute, before being attr encoded.
Carousel: Add and use an in_gallery variable
set by a low-priority post_gallery hook

Props @chrisbliss18
Carousel: fix check if we're in a gallery
Shortcodes: Fix [googlemaps] shortcode reversal false positive.

`jetpack_googlemaps_embed_to_short_code()` is intended to only reverse URLs in
the format of `https://maps.google.com` or `https://google.com/maps/`, but it
is matching `google.com` any domain.

For example, the following tag will be reversed into a `[googlemaps]` shortcode
even though it should not be:

<iframe src="https://docs.google.com/spreadsheets/d/19FacgevvN-GGS8SOlbMRVKkWDYSMDQEBfRmkwqaA9z8/pubhtml?widget=true&amp;headers=false"></iframe>

The problem is that the code mistakeningly tests for a `false` return from
`preg_match()` instead of `0` or `false`. Unlike `strpos()`, `preg_match()`
returns `0` if the pattern is not matched, and `false` if there is an error.
Because of this, the condition evaluates to true, and the function
doesn't returns early.

This bug was introduced in 7d5c5a77b25474798554fba4e8f72c853f96b507.
Sharedaddy: only enqueue Javascript when Shares are displayed

Check our sharing options: if the sharing buttons are set to appear on that page, we enqueue the js
Sharedaddy: move check if sharing is enabled to its own function
Sharedaddy: only enqueue CSS if the buttons should be displayed
Sharedaddy: always enqueue scripts on themes supporting IS
Sharedaddy: align statements
Sharedaddy: add filter to change when files are enqueued
Sharedaddy: only enqueue files when IS is active
Sharedaddy: remove IS from conditional

IS already takes care of enqueuing resources if needed
Sharedaddy: change $global into $global_options

To avoid any confusion with GLOBALS
Change LinkedIn sharing URL to secure URL

LinkedIn supports https throughout their site and users should be sent
to https links when possible. This commit changes the Share_LinkedIn
class to use https for the sharing link used in the process_request()
method.
Reverting some unintentional whitespace changes
Related Posts: add filters to customize post context

See #361
Contact Form: add required and aria-required attributes

Fixes #459
Using $this->http() to make http usage consistent

Switching from a hard coded https in the LinkedIn class to using
the http() method to make the prefix usage consistent across the
methods and classes.
Shortcodes: add shortcode param to all shortcode_atts calls

See http://wp.me/p56-127
Fixes #527
Add `absolutize_css_urls` function and unit test.