Add <html> element to layout

Every HTML document should have an <html> element as its root node.

Change-Id: I20e13d129cd08619d2e5b87e23e884f3f014aa2e
Set <html> lang attribute to I18n.locale

Some information from the W3C on this attribute can be found at:

- http://www.w3.org/TR/2014/NOTE-WCAG20-TECHS-20140311/H57
- https://www.w3.org/International/questions/qa-html-language-declarations
- https://www.w3.org/International/questions/qa-lang-why

Change-Id: Icfbc76c87c003e191323dfcc869607a6970fd7e7