Don't show the VaultPress module as active.

Unless, y’know, the plugin is active.
Whitespaaaaaace
Show VaultPress in the modules listing page.
VaultPress shouldn't be activateable.
Don't display VaultPress at a lower opacity.
VaultPress never gets deactivate link.
Merge pull request #1189 from Automattic/tweaks/vaultpress

Modifications to how the VaultPress module behaves.