Make the config a singleton

...hub.com/turt2live/matrix-media-repo/client/r0/download.go

...ub.com/turt2live/matrix-media-repo/client/r0/identicon.go

....com/turt2live/matrix-media-repo/client/r0/preview_url.go

...ub.com/turt2live/matrix-media-repo/client/r0/thumbnail.go

...ithub.com/turt2live/matrix-media-repo/client/r0/upload.go

...om/turt2live/matrix-media-repo/cmd/import_synapse/main.go

...ub.com/turt2live/matrix-media-repo/cmd/media_repo/main.go

src/github.com/turt2live/matrix-media-repo/config/config.go

....com/turt2live/matrix-media-repo/rcontext/request_info.go

...atrix-media-repo/services/handlers/opengraph_previewer.go

...x-media-repo/services/handlers/remote_media_downloader.go

...t2live/matrix-media-repo/services/handlers/thumbnailer.go

...com/turt2live/matrix-media-repo/services/media_service.go

...turt2live/matrix-media-repo/services/thumbnail_service.go

...b.com/turt2live/matrix-media-repo/services/url_service.go

...hub.com/turt2live/matrix-media-repo/storage/file_store.go

...ub.com/turt2live/matrix-media-repo/synapse/local_media.go

src/github.com/turt2live/matrix-media-repo/util/config.go

...m/turt2live/matrix-media-repo/util/errcodes/errorcodes.go

src/github.com/turt2live/matrix-media-repo/util/matrix.go