Now that images are in a database and we can make filtered queries, we can do away with the cumbersome `resultsSlice` and `uploadsSlice`.
- Remove `resultsSlice` and `uploadsSlice` entirely
- Add `imagesSlice` fills the same role
- Convert the application to use `imagesSlice`, reducing a lot of messy logic where we had to check which category was selected
- Add a simple filter popover to the gallery, which lets you select any number of image categories
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