Export/Import Group Shares

[IljaN]

This has more implications as observed in #6.

Let's use this ticket to discuss strategies we can use, considering that group federated shares are not possible.

The most complex thing we could do is convert group shares to individual user shares during the migration, then back again to group shares when all is done. This would be very complex and I'd prefer avoiding this.

The simplest approach is agreeing on the limitation that group shares might not work during a partial migration and inform the admin appropriately.

The minimum task needed here is the ability to export and import the share data, without any rewiring/mapping.

A medium approach would be to inform the admin what users from what groups they need to migrate in what order to keep the system functioning as long as possible (with possible short disruptions if no perfect scenario is found).

The extended task is thinking of approaches to help the admin keep the system functioning during migration.

@jvillafanez @IljaN

@felixboehm I'm not sure whether you were aware of this limitation when you suggested the user share rewiring, so adding you here in case you didn't and might have had other ideas in mind.

[felixboehm]

can't see what is open here. Shares should work already, also group shares.
We only handle groups existing in target system, so this should work:

1. create groups
2. import shares