(Bug 2165) Revamp community posting options

* "All members" now grants posting access to *all* members, instead of
  still allowing you to selectively grant/take away access. This applies
  retroactively (so everyone, including those who didn't explicitly have
  posting access before, can now post to the community)

* "Select members" acts as described: you can choose which of your
  members has posting access

* New option: "new members can/cannot post" which is only visible when
  "select members" is selected. "New members can post" is like the old
  "all members" option. "New members cannot post" is like the old
  "select members" option. Default is for new members to be unable to
  post unless; they have to be given posting access
Merge pull request #648 from afuna/community-posting-options-cleanup-bug2165

(Bug 2165) Revamp community posting options