feat: implement parent tag search

[Computerdores]

Searching for a tag now includes entries that are only tagged with child tags of the tag. A search for Nintendo would therefore also include entries that only have the tag Mario or Luigi (assuming that Nintendo is a parent tag of Mario and Luigi).

Close #661.

[CyanVoxel]

Seems to function as it's supposed to! The query times can get quite high for some larger libraries and tag sets (16+ seconds for me), but I suppose that's the nature of a non-indexed recursive search like this?

[python357-1]

I think this is good for merging but we should probably take a look at the performance when we can, preferably before release. Cyan is probably at the higher end in terms of tags in a library but if a user gets search times near 16 seconds, they will probably think something is wrong.