Dashboard restyling (#6)

* Dashboard restyling to align it with the Jetpack branding

* Refactor a bunch of things to use and reuse the same wrappers for all views. Add notices. Add footer. Put fatal error into a warning notice.

* Stylize all UI messages consistently

* Refactor to render the main content first without outputting it, and save whether the link to the VaultPress online dashboard must be shown or not

This data is luego used in the caller function that handles the general rendering

* Introduce the Jetpack logo package and place it under the footer

* Use the gray version of the Jetpack logo

* Move notice styles to a new stylesheet so they have a consistent look throughout the wp admin. Add classes so the notices are moved by WP core below the heading

* Add a prompt so user has to confirm that they want to reset VP options in database

* Set correct alert level for notices

* Fix issue where I reverted a condition to test and didn't put it back as it was.

* Fix wrapper width on large screens. Make cards stack on small screens.

* Link the Powered by Jetpack to the Jetpack dashboard if it exists or jetpack.com otherwise

* Hide heading served remotely in certain UI messages

* Fix offset layout when Jetpack is deactivated. This is because when JP is active there's a different class in the body than when it's inactive and VP stands on its own admin screen

* Fix layout of top notice on mobile

* Hide notices that are shown above the dashboard wrapper just like we do in Jetpack