db endpoints for user modification

This is adding some db access methods to allow modification of user account data. I basically copied over the workflow from chishop and I factored along the way.

Methods include:

* insert_user
* update_user
* delete_user
* gpg key / otk / email modification methods

https://bitbucket.org/pypa/pypi/src/60c336a587ab4ba97c758f35bf1c3cc716a18029/store.py?at=default