Expand "Settings" UI to full-width

Summary:
Depends on D19988. See D19826 for the last UI expansion. I don't have an especially strong product rationale for un-fixed-width'ing Settings since it doesn't suffer from the "mystery meat actions" issues that other fixed-width UIs do, but I like the full-width UI better and the other other fixed-width UIs all (?) have some actual rationale (e.g., large tables, multiple actions on subpanels), so "consistency" is an argument here.

Also rename "account" to "language" since both settings are language-related.

This moves away from the direction in D18436.

Test Plan:
Clicked each Settings panel, saw sensible rendering at full-width.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20005