AtD: jshint

Merges r98088-wpcom.
Contact Form: Switch the method by which we determine what numeric index new options should be given in the wizard.

The other method didn't account for a difference in how the fields are built when the field is brand new vs. when you're editing a form that already existed. In the former case, there's an element with a different that wasn't being included in our count when the field was brand new and which accordingly also necessitated the check for zero length that I here delete. Now we latch onto a different classname that selects both the first option and any subsequently added ones.

See http://[private link]

Merges r98059-wpcom.
REST Proxy: Cache bust the JS files that load the iframe.

http://[private link]

Merges r98077-wpcom.
SoundCloud: Correctly serve embed over HTTPS.

Without the slash, SoundCloud redirects to a URL with the slash, but without HTTPS.

{{{
$ curl -I 'https://w.soundcloud.com/player?url=https%3A%2F%2Fapi.soundcloud.com%2Ftracks%2F150803269&auto_play=false&hide_related=false&visual=true'
HTTP/1.1 301 Moved Permanently
Location: http://w.soundcloud.com/player/?url=https%3A%2F%2Fapi.soundcloud.com%2Ftracks%2F150803269&auto_play=false&hide_related=false&visual=true

$ curl -I 'https://w.soundcloud.com/player/?url=https%3A%2F%2Fapi.soundcloud.com%2Ftracks%2F150803269&auto_play=false&hide_related=false&visual=true'
HTTP/1.1 200 OK
}}}

Merges r98032-wpcom.
Vimeo Shortcode: Load iframe over HTTPS when embedding page is served over HTTPS.

https://[private link]#comment-23768

Merges r98052-wpcom.