put media in filesystem

• I agree to release my code and all other changes of this MR under the Apache-2.0 license

Currently I removed media from database and put them in db/media/<base_64_of_key> but it is not streaming because Vec<u8> of requested file is already in the memory. Do we need change ruma api or we should put away ruma for media part? If changing ruma is better, I can follow that in upstream and we can merge this until ruma api changes because this can reduce ram usage alot on it's own (sled won't cache media in ram) and open doors for streaming.

cc: #93 (closed)