Multisite: Activate modules when new sites are created.

If Jetpack is on a single-site installation, or is not network-activated in a multisite installation, then modules get activated automatically when the plugin is activated.

In a Multisite installation where Jetpack is network-activated, though, that doesn't happen unless we trigger it inside a wpmu_new_blog callback.
Multisite: Correct module slugs used on Network Settings screen.

Previously the module slugs were derived by running the module name through a few string manipuations, but this resulted in incorrect slugs in some cases. For instance, it resulted in "shortcodeembeds" instead of "shortcodes", and "contactform" instead of "contact-form".

Because of this, the modules failed to be activated when new sites were created because the incorrect slug was passed to activate_default_modules(), and it couldn't verify that the associated module file existed.

This commit gets the canonical slugs from get_available_modules() instead. As a result, list_modules() and its associated unit test are no longer needed.
Fix merge conflict.