Add support nested groups on the Repositories page

Add support nested groups on the Repositories page

Fix for #725.
Also covers #527.

This is a squashed commit of the following commits,
merging and closing pull request #1267:

commit 55fee41769ffab1aff59344fe117d481687aa743
Author: Martin Spielmann <mail@martinspielmann.de>
Date:   Mon Nov 6 17:19:53 2017 +0100

    declared local variable final to fix travis build

commit 131e4d14a48c2a3fdce621fa54637de50684d040
Author: Martin Spielmann <mail@martinspielmann.de>
Date:   Mon Nov 6 14:11:55 2017 +0100

    fix formatting (use tab for identation)

commit 8da5f6d5967894f157251c320928acdab3a451e7
Author: Martin Spielmann <mail@martinspielmann.de>
Date:   Mon Nov 6 13:45:39 2017 +0100

    Add repositoryListType tree. Addresses #725, 527 and includes #1224

commit 6c061651fb95212ae242dbca06c8d9ef80146201
Merge: f365daa3 40ee9653
Author: Martin Spielmann <mail@martinspielmann.de>
Date:   Sat Nov 4 13:19:08 2017 +0100

    Merge remote-tracking branch 'collapsible/ticket/527' into 725_nested_repos

commit f365daa3b1d6be135365f9b11bdece320beabf4e
Author: Martin Spielmann <mail@martinspielmann.de>
Date:   Sat Nov 4 13:10:24 2017 +0100

    first working version of tree model