Add recurring tasks to purge removable content

cmd/media_repo/main.go

cmd/media_repo/reloads.go

common/config/conf_main.go

common/config/models_main.go

common/config/watch.go

common/globals/reload.go

config.sample.yaml

docs/config.md

storage/stores/thumbnail_store.go

storage/stores/url_store.go

tasks/all.go

tasks/purge_previews.go

tasks/purge_remote_media.go

tasks/purge_thumbnails.go