Improve Login page

* Use mw-ui-checkboxes (follow MW Login Ib585e32463765d7eb7d0be5dae54dc828ddd4ca9)

* Seperate GoogleLogin area and MW Login are with a "or" line to prevent
confusion (especially the second "keep logged in")

* Use ResourceLoader module to load style instead of OutputPage::addStyle

Change-Id: I62300f0d6ace52a03b28e51be642ae04da684f2e