Add ability to perform one-time replication to legacy UI

Docs verified in legacy UI version of Guide.

Initial docs PR: https://github.com/freenas/freenas-docs/pull/1167

One time replication totally makes sense for me - I'll try to include this idea in our new replication code.

Commenting on manual replication triggering, it could make a lot of sense in some contexts:

• Servers and datasets which are used on specific projects, or periodically, rather than continually being updated on an ongoing basis, that don't need a periodic replication task; they only need replication at some specific points judged by the user. It's true that if nothing changes, nothing will be replicated, but running a pointless task continually when the user can judge when it's actually needed might be preferable to some people.

• Manual triggering might also be desired due to fitting around timing of other events - for example if it has to be run early due to circumstances affecting the next day, or if the best timing is inherently variable from day to day (run it during the end of day meeting, or when watching a movie whenever that is, so we can check it completed before we turn off and go home or go to bed?).

• If replication is used to create ad-hoc duplicates as needed, for example deploying a duplicate of a file server, or mirroring latest data to a set of HDDs to take and import when travelling away somewhere that the main server won't be accessible, or where the amount of data in the delta for this replication would be unusually large and impractical to send other than by manually replicating to clean disks and sending those instead.

If I understand your question correctly here is an example

Site 1:
Host (linux with ZFS) <--> FreeNas1

Site 2
Freenas2

Host syncs its data with FreeNas1 via ZFS push/recv (datasets and all snapshots)

Now I'd like to sync both FreeNas boxes with build sync functionality but fail (currently it impossible to create sync job with out snapshot task - but I don't need snapshot task on FreeNas1 because all the snapshots are created and maintained during sync with host)