Twenty Thirteen: Cover block alignment issues

[melchoyce]

The cover block positioning is broken in Twenty Thirteen. Centered content is offset, and trying to use the new block alignment tool positions content off-screen.

Attaching screenshots to demonstrate.

[melchoyce]

Using the "bottom left" alignment option in the cover block

[melchoyce]

Default, center alignment

Trac ticket: https://core.trac.wordpress.org/ticket/51224

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This PR clears up the front-end display of cover block children. To match the editor, they have their margins set to auto, and are always the default width.*

_*I find this part weird, but that's how these appear in the editor, so I figured let's match that._

Test content: ​https://gist.github.com/kjellr/cb83095e95b28cc914e970ca0c7f7b18

## Screenshots

_Before:_

_After:_

_Editor (for reference):_

Seeing some weirdness in the placeholder still:

Everything else seems to be working 👍

That's the Group block placeholder, right? I hadn't touched that block in this PR, so I think that's a separate bug. I took a quick look now, but the fix isn't obvious, so I suggest we punt that to a separate bug report + PR.

Sounds good 👍

[ryelle]

In 50346:

Twenty Thirteen: Fix alignment of child blocks within the Cover block.

Props kjellr, melchoyce.
Fixes #51224.

Committed in r50346.