Bunch of minor JP admin css tweaks

- Fixed left aligned image bug in modals at mobile resolution.
- Decreased excess padding/margin around clouds at mobile resolution.
- Show activate/deactivate links in mobile view, works on hover in
desktop
- Fixed margin-top on filter button in mobile view
- Fixed white horizontal line when left nav menu is collapsed
- Changed message on homepage to no longer mention, “Once you’ve
connected Jetpack…” if you’re already connected.
Merge pull request #502 from Automattic/ui-tweaks-dave

Bunch of minor JP admin css tweaks