My Blogs Post Editor: Add Preview button. Add revision support to API.

Merges r101435-wpcom.
Likes: update the Settings > Discussion > "Someone likes one of my posts" toggle to appear enabled by default.

This feature is already enabled by default, but this option made it appear as though the feature is disabled
in certain situations. On blogs where the settings on the Discussion page haven't been updated (e.g. a new
blog), the absence of a saved value for this option was causing the Likes notifications to be sent -- even
though this option appeared disabled on the admin page.

https://[private link]

Merges r101245-wpcom.
Publicize: allow html entities to be part of the messages sent to services without allowing actual html tags

Standardize the way we sanitize the messages sent to services into a single method `Publicize_Util::sanitize_message`, which first removes `script` and `style` tags, then runs the content through kses, trims whitespaces and decodes any left over entities.

This allows users to have hearts and other symbols in their publicize messages. This is a redux of [101294] which was reverted in [101468] because it was causing html to be displayed in publicized posts.

The following code has been tested to ensure that the problems from before are no longer present.

see https://[private link]
see https://[private link]

Merges r101489-wpcom.
Sharing Services/Shareddady: remove support for Digg who's API no longer works

see https://[private link]

Merges r101457-wpcom.
Sharing Services/Shareddady: remove the Share_Digg class now that the service is no longer loaded

see [101457]
see https://[private link]

Merges r101459-wpcom.
Digg: disable the shortcode since the API is no longer working

see https://[private link]

Merges r101456-wpcom.
REST API: when rendering embeds provide any needed scripts and styles in the response

Moves the code used for determing scripts/styles for shortcodes into a new helper class that is now used for both shortcodes and embeds

see https://[private link]

Merges r101562-wpcom.