deps: bump boorufetch due to danbooru breaking tag categories

fetch: retry on API throttling

only fetch new pages fromm danbooru, if there are too few pending images

deps: update to opencv4

client: fix submit button disappearing

lowercase all DB tags and fix infinite loop

go: bump package version

switch fully to Danbooru

gelbooru: improve blacklisting

gelbboru: blacklist image, if danbooru tags don;t contain requested tag