Issue #1810472 by linclark, mradcliffe: add Symfony's Serializer component to core despite Symfony potentially releasing BC-breaking updates after 2.3.

This reverts commit f8673ac4.

Needs more discussion.

Issue #1798732 by Dean Reilly, westie, alexpott, sun: convert install_task() variable to use state system.

Issue #1812034 by xjm: Only use standard profile where necessary in comment.module tests (10% bot speed-up).

Dang it. I effed up commit credit. Reverting to I can re-commit again.
Post-push git commit --amend support on d.o would be so lovely.

This reverts commit 722432c3.

Issue #1787846 by Gábor Hojtsy, EclipseGc, effulgentsia, fubhy: Added Themes should declare their layouts.

Issue #1792600 by Stalski, fubhy, swentel: Refactor field_ui() so common behavior for fields and display overview screens are extracted.

Issue #1149866 by jessebeach, effulgentsia, ethanw, corbacho, et al: Added Backbone.js and Underscore.js to core.

Issue #1810990 by sun, chx, Berdir: Fixed Teared down test environment leaks into parent site/test runner.

Issue #1723892 by Berdir, Pancho, das-peter, fago: Support for revisions for entity save and delete operations.

Issue #1277352 by quartsize, lewisnyman, nod_, JohnAlbin, kika, dcmouyard: Fixed Responsive vertical tabs.

Issue #1775780 by mr.baileys, -enzo-, kbasarab, alexpott, sun: Fixed Needs tests: Forum module's block configuration for block.active.limit and block.new.limit .

Issue #843114 by sun, quicksketch, Berdir, c960657, Crell: Fixed DatabaseConnection::__construct() and DatabaseConnection_mysql()::__construct() leaks  (Too many connections).

Issue #1808864 by fastangel, Jose Reyero: Fixed Exception handling is not 'locale safe' (thus not database safe).

Issue #1810652 by xjm: Fixed admin/config/system/site-information still references the default content feed when node.module is disabled.

Issue #1798732 by Dean Reilly, westie, alexpott: Convert install_task() variable to use state system.