Added `Sequence` widget

Summary:
Yo dawg, I hear you like multi-editors in your multi-editors!

Note: When editing a sequence, you can't see all of your content.
Also, can't see negative left margins on article.html:Preview so you
won't see the green checkmarks there.

copypasta

Test Plan: article.html

Reviewers: aria

Subscribers:

Differential Revision: http://phabricator.khanacademy.org/D14232
2 things that look pretty on the page but ugly in code.

Summary:
Center paragraphs in articles that contain only images or only
widgets.

Test Plan:
i tested a little bit on my particle. mostly i'm

tscrossedfingers

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D14227