Loader: Subset groups using client operations

[iLLiCiTiT]

Brief description

Use client opeartions to change subset group on subset documents.

Testing notes:

1. Open any host on project where you have something published
2. Open loader tool in the host
3. Select subset/s
4. Hit Ctrl + G
5. Change group of subset
6. Group change should be propagated

[ynbot]

Task linked: OP-3813 Subset groups using operations

[BigRoy]

Wouldn't next make more sense here since you always just want the first entry?

[iLLiCiTiT]

To be honest I don't like using next for such long expressions especially when it would make it even longer.

repre_info = next(
    self.sync_server.get_repre_info_for_versions(
        project_name,
        [version_doc["_id"]],
        self.active_site,
        self.remote_site
    ),
    None
)