Possible performance improvement: add indexes per category

[tophf]

Problem: the current index is like 20MB gzipped so it may take half a minute to download on a slower connection.

Idea:

• add individual indexes per each category - will be used by Stylus in the popup and by the site when the tab is created using a direct category link; Stylus may also download the global category index (currently ~10k styles) in case the corresponding option is enabled in the popup

• for https://uso.kkx.one/browse/styles add 5 tiny indexes with 36 styles inside for each of the 5 sorting modes - will be used to display the first page instantly, while all of the category indexes are being downloaded.

• for https://uso.kkx.one/browse/categories - add an index of category names and an index for the first page results