can't PUSH to pocket server

[mfrasca]

several mistakes slipped into 1.0.87 pocket server.

• [ ] current_user is untested
• [x] Verification is not available neither in bauble/plugins/garden/__init__.py nor, consequently, in bauble/plugins/garden/import_pocket_log.py
• [x] field is_exporting is not set until a snapshot is exported.
• [x] current_user is not defined
• [x] python2.7 has no "xb" mode and no FileExistsError exception.
• [x] correct head-tail splitting, so easy in python3