Add network mode

[krafit]

If active on a network level in WordPress, AntiVirus should be able to run gobally rather than / not only on each site

Currently it can be activated on a network level but runs only in the indivual dashboards.

[2ndkauboy]

On a network level (or even in a single side) it might be a good idea to allow scanning of any theme, not just the current one. This might be combined with the need to scan both child and parent for a site.