Detected media should be auto-inserted when Pressing a service/domain-specific page

[stephdau]

If a user is "pressing" a page from a service we know and support, we should auto-embed said media on pageload.

Attached is a patch that does this (for embeds, for now, not images) for pages like:

• ​http://www.youtube.com/watch?v=XFFdoH_AeQE (or ​http://www.youtube.com/watch?whatever=blah&v=XFFdoH_AeQE, etc)
• ​https://vimeo.com/118458120 (or ​https://vimeo.com/channels/staffpicks/118458120, etc)
• ​https://soundcloud.com/frankiebones/live-output-brooklyn-ny-may-23-2014
• and so on

Note the update to WP_Press_This::get_canonical_link() to support them if from root, instead of fully qualified.
EG: Vimeo: <link rel="canonical" href="/118458120">

[stephdau]

Even simpler way to define domain/service-specific pages.

[azaozz]

Thinking this should happen for all oEmbed sites we support. If the URL would trigger an oEmbed the same way as if pasted in the editor, we should just add it there with no other content?

[azaozz]

In 31827:

Press This: check if the URL is to embeddable content on a site that supports oEmbed. Then insert only the URL in the editor. It will trigger a wpView with the embedded content.
Fixes #31637.

[azaozz]

In 31836:

Press This: small clean-up when generating the suggested content. Ensure all keys are always set in the array that is passed through the filter.
See #31637.