Hide placeholders and table headings until the table is loaded

To indicate the table is loaded, remove 'js-not-loaded' class name from the stats table

Pick notes: We already had picked later changes, so just get the missing bits,
    and avoid picking changes that will be dropped in later commits (that we
    already have picked).

Conflicts:
	pootle/apps/pootle_misc/browser.py
	pootle/static/css/style.css
	pootle/templates/core/table.html