Improve media download rate limiting (#586)

CHANGELOG.md

api/r0/download.go

api/r0/thumbnail.go

api/webserver.go

cmd/workers/media_repo/reloads.go

common/config/conf_main.go

common/config/models_main.go

common/config/watch.go

common/errors.go

common/globals/reload.go

config.sample.yaml

go.mod

go.sum

limits/leaky_buckets.go

limits/rate_limiting.go

pipelines/pipeline_download/pipeline.go

pipelines/pipeline_thumbnail/pipeline.go