Move authentication handler out of the endpoints

src/github.com/turt2live/matrix-media-repo/api/auth.go

...github.com/turt2live/matrix-media-repo/api/r0/download.go

...ithub.com/turt2live/matrix-media-repo/api/r0/identicon.go

src/github.com/turt2live/matrix-media-repo/api/r0/info.go

...thub.com/turt2live/matrix-media-repo/api/r0/local_copy.go

...hub.com/turt2live/matrix-media-repo/api/r0/preview_url.go

src/github.com/turt2live/matrix-media-repo/api/r0/purge.go

...thub.com/turt2live/matrix-media-repo/api/r0/quarantine.go

...ithub.com/turt2live/matrix-media-repo/api/r0/thumbnail.go

src/github.com/turt2live/matrix-media-repo/api/r0/upload.go

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