Adding theme enable/disable commands for multisite installs. Closes #918.

These commands allow the user to enable or disable themes for a site in a multisite network, or network enable/disable themes.
Ensuring that theme enable/disable commands are only ever run on multisite installs
Adding feature tests for `theme enable` and `theme disable`
Testing that themes are actually being en/disabled
Merge pull request #938 from mboynes/network-theme-commands

Adding theme enable/disable commands for multisite installs. Closes #918.