Make import list filterable

[stefan-niedermann]

When the list of importable accounts is displayed, it would be great to be able to filter the ones out that are actually imported already.

workflow in 3rd party app like deck or news basically would be:

display dialog with filter = []

-> displays user A and user B

import user A

display dialog with filter = [A]

-> displays only user B

import user B;

cc @nextcloud/designers