Unify permissions dialog for network posts, channel posts, and remote posts
Created by: Treer
Adds the warning and help link to Rpost.php, and has all 3 files read the warning string from the same place instead of duplicating it (I put it in acl_selectors.php, but can move it if there's somewhere more suitable)
Also
- changes the warning from being about when a post is "sent" to when it's "shared", to match the Share button.
- hyperlinks the "cannot be changed" part of the warning to the help file
- adds some help