Menu visibility: add "postAuthor" as a potential value for `forUserType` on menu items

[fsvreddit]

Right now, the only options for menu item visibility is via forUserType and the options are relatively limiting.

I would ideally like an option for forUserType: "postAuthor" to be available on both posts and comments because I can think of many situations where an OP-only menu item would be useful - such as for the OP to award reputation points in ReputatorBot.