This commit implements the backend of migrating the
`allow_edit_history` setting to
`message_edit_history_visibility_policy`.
This allows organizations, to have an intermediate setting to
view only the "Moves" history of the messages.
We still pass `realm_allow_edit_history` in `/register` response
though for older clients with its value being set depending on the
value of `realm_message_edit_history_visibility_policy`. We set
`realm_allow_edit_history` to `False` if the
`realm_message_edit_history_visibility_policy` is "None", and
`True` for "Moves only" or "All" message edit history.
Fixes part of #21398.
Co-authored-by: Shlok Patel <shlokcpatel2001@gmail.com>
Co-authored-by: Tim Abbott <tabbott@zulip.com>
