Display poster column only when relevant

* in members/new, only show the poster column if the community post
 level is "select", instead of always showing (that was left over from
 when you could always revoke poster access, even if the post level was
 "all")

* in members/edit, always show the poster column if anyone has "posting
 access" set on them (this makes it act the same as
 unmoderated/moderator columns). This is a bit of an edge case, caused
 when someone switches community settings, but their community members
 used to have posting access explicitly -- this meant that the members
 would still show up on the "edit community members" page, because they
 still had an edge connected to the community, but there might not be a
 checkbox to uncheck