Add language selector to dashboard
Adding styling to form and submit button
Display language selector conditionally

Add a constance setting that controls whether we display
the language selector on the dashboard. Defaults to off.

(Modify setting at /2014/admin/constance/config/)
Add doc for language selector
Merge branch 'feature/optional-language-selector' into develop
Merge branch 'develop' into feature/78-refactor-settings
Cleanup, comments, tweaks