Multisite: Added pagination to sites page

Added pagination to Network Admin > Jetpack > Sites page. Defaults to 40 items
per page.

Resolves #309
Multisite: Removed Jetpack column from Sites list

The content of the Network Admin > Sites Jetpack column was removed before
the last release. The column itself managed to slip into the release. This
commit removes it.
Multisite: Load code only where needed

The filter for actions from the Network Admin > Jetpack > Sites page was firing
and loadking all the code handling actions for that page on every page of the
Network Admin. This only loads code on the page it is needed on.