Share: filter existing user/group from suggestion

[tobiasKaminsky]

• have a file shared with "admin"
• share the file and search for "admin"

-> currently you get "admin" and can select this, but nothing happens

Instead it should filter out "admin" as it is confusing to reshare the file to a sharee which already exists.

-> I will do this after #2573

[stale]

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