Admin panel: Add Vocabulary item list view

[SarahW91]

Implement vocabulary item list view as described in https://codimd.web.cern.ch/UTnvE-L2RrSar3_szWWK_w

[SarahW91]

I still have a few issues with this view:

• Multiple columns are shown for all possible name/title variations. It would be nicer to select the appropriate one(s) depending on the vocabulary type.
• Title is always used in English instead of preferring the instances selected language.
• Search only works via ID of an item, not the displayed name/title.
• Sorting by anything other than the displayed name/title leads to an error (and sort doesn't work in the UI anyways, does it?)

[SarahW91]

@slint Most of my questions were resolved with the changes we discussed last week. However, the item search is still a bit weird to me. Searching for partial names/titles/subjects sometimes leads to results, sometimes not. I don't quite understand why.
Another thing is the localization of the displayed title. I'm not quite sure if this can be done or how.