Sketch of saveCenter UI. Refs #279, #836.
Use same saveCenter ui in source editor.
Add tests for behavior of js-mapCenter, js-savedCenter, js-saveCenter
Rename js-saveCenter to js-lockCenter for less braintwists.
Merge branch 'mb-pages' into saveCenter
Set changed flag when saveCenter lock is changed.
Merge pull request #842 from mapbox/saveCenter

Save center UI