Add "moderator on network" option to category permissions

[cellio]

Categories can restrict visibility, posting, or both. This allows Meta to have a blog that only staff can post to, allows Judaism to have a Purim Torah category that is hidden most of the year (it's a seasonal thing), and so on. The options for restrictions are:

• everyone
• anyone with a user account
• all but new users
• veteran users
• moderators only
• staff only

It would be helpful for our instance to have a place where moderators from across the network could ask questions or share information. Other deployments could benefit from this too; for example, a university that has communities for each department or college might want to have a place where all faculty can interact.

Let's split "moderators only" into two options:

• moderators on this community
• moderators anywhere on the network

Terminology note: QPixel has the concept of a "network moderator", meaning somebody with moderator status on all communities. This is not that. The goal here is "any", not "all".

Instead of just checking the current community, the new option would need to look up the user's status on all communities and then allow entry if the results contain a match. Everything's in one database and I think in one table, so I don't think this is expensive.