Toolbar button to select all items in a view

[tnajdek]

https://forums.zotero.org/discussion/115630/ctrl-a-isnt-working-for-me-in-zotero-web

This would most likely need to fetch all items (#444) or at least the keys (which might be what we end up doing for #253). Having this would enable #443. It would definitely require improvements to "ongoing" (at least #478).