UNICORE storage explorer

Refine digital object interface with focus on displaying Unicore storages in the same view as data sources. Closes #611 (closed)

The following things are implemented now:

• Implementing interface for UNICORE storages
• Implementing interface for SSH access
• Showing directories
• Downloading files
• Tagging files and directories (metadata storage; only dynamic attributes needed)
• Link to URL where directories are displayed (actions; only name, link, icon needed)
• Edit SSH data sources separately (related to #476)
• Breadcrumb menu (manual building is no longer possible; #486 (closed) is needed)
• Test/Fix showing UNICORE storages.
• Check if all new functionality has doc strings / Write some tests
• Add notice to changelog