[#731] Remove post-dating checks on posting, allow any time

  * removed the "are you posting in the past" check. Always allow
    entries to be posted regardless of the date on their entry

  * checking the "don't show on reading pages" checkbox now just
    prevents it from showing on reading pages (instead of having to be
    used to avoid the above error)

  * tweaked the labels to remove mention of date out of order

  * still using the old (opt_backdated) property in the backend
Merge pull request #731 from afuna/unenforce-backdate-bug-731

Remove past-dating checks on posting, allow any time