Handle ambiguous annotation values

[bkis]

• anno values can be str or list[str]
• search index does the same, gets "foo" or ["foo", "bar"]
• search UI anno value select is multi-select
• content UI anno value select is multi-select

[bkis]

Actually, for the search it'd be enough (or even better for UX) to keep the single value select/input. It would then find annotations matching this value and ones with ambiguous values would match one of the possible values. You know.