pzh login mobile/tablet fix + desktop recenter

jira issue: WP-943

the css change is a fix for the desktop (the box wasn't properly centered) + mobile version media query
the main thing was the "meta" - it should look more properly on a tablet right now, but if you consider it still too small, let me know (but changing it further might be difficult without breaking stuff - css might affect desktop, and setting specific dimensions in the meta is not really elegant) - I'll try to come up with something if needed.