Simplified button style declarations

These are now pretty specific and several types and sizes of buttons are
available. Further reuse should be easy to implement whenever CSS
preprocessors are made available.

Some notable style changes have been done too, but the core idea is having
more reusable components.

Conflicts:
	pootle/static/css/style.css
	pootle/templates/admin/reports.html
	pootle/templates/admin/settings.html
	pootle/templates/auth/link_with_evernote.html
	pootle/templates/login.html
	pootle/templates/profiles/settings/evernote_account.html
	pootle/templates/registration/password_change_form.html
	pootle/templates/registration/password_reset_confirm.html
	pootle/templates/registration/password_reset_form.html
Followup: Apply new button styles to the rest of the templates