Remove auto-embed option, enable it always and make it a filter

[nacin]

When embeds were introduced in #10337, an option for turning off (enabled by default) auto-embeds was added without much discussion as to whether the option was actually necessary.

We should remove it. Our oEmbed implementation is pretty strict, requiring [embed] tags or for the URL to appear on its own line, and be whitelisted, so there aren't any valid concerns about accidentally messing with someone's content. Especially with Twitter embeds really getting a lot of attention in 3.4, this option is just cruft and gets in the way of users trying to embed things.

Along with #21507, I get another half-credit for removing an option. (“Add one, remove two...”)

[nacin]

"Maximum embed size" doesn't really make much sense anymore, without the context of "Auto-embeds" and "When possible, embed the media content from a URL directly onto the page". I think I'm for removing it as well, defaulting to $content_width, and 600px for a height. (It is already filterable.)

[obenland]

Thought I'd try to come up with a patch.

Two things I wasn't sure about:

• Should we keep calling do_settings_fields('media', 'embeds'); for back-compat underneath 'default'?

• Do we do ternary operators in array values? Didn't say anything about that in ​the handbook.

[wonderboymusic]

I don't know why Nacin made 2 tickets but his most recent comments are on #21719