[refactor] move server blobs to its own submodule

src/leap/soledad/server/_blobs.py → src/leap/soledad/server/_blobs/__init__.py

 # -*- coding: utf-8 -*-
-# _blobs.py
+# _blobs/__init__.py
 # Copyright (C) 2017 LEAP
 #
 # This program is free software: you can redistribute it and/or modify