Revert "Revert "Bug 1042744 - Add a container around app iframes and some chrome UI. r=alive" under suspicion of breaking JB emulator builds"

This reverts commit ebb824c7e4c1be271d68c773ac75ae0cfb79238d.
Merge pull request #22200 from vingtetun/revert.revert.browser-container

Revert "Revert "Bug 1042744 - Add a container around app iframes and som...
Bug 1020306 - Make the thread/message edit mode to match the style used by other applications