jshint: Apply some very safe changes.

Merges r100305-wpcom.
REST API: fix an entity encoding bug

Merges r100167-wpcom.
REST API: fix the same entity encoding bug as in [100167] but this time for the `edit` context too

Merges r100170-wpcom.
REST API: when creating a post if the post type is post, there's no title set, and there is media, we automatically use the first line of content to set the title.

see https://[private link]

Merges r100173-wpcom.
REST API: when creating a post if the post type is post, there's no title set, we automatically use the first line of eligible content to set the title.

This is a further tweak to [100173]

see https://[private link]

Merges r100175-wpcom.
REST API: revert the changes from [100173] and [100175] -- doesn't work as expected in production

see https://[private link]

Merges r100177-wpcom.
Media summary: Facebook doesn't like that `youtu.be` URLs have a redirect in them, so we now parse them separately and construct the `youtube.com` URL for that video so that there is no exta redirect

see https://[private link]

Merges r100179-wpcom.
jshint: Unignore a few files