default_query: write ops are now supported
API: Fix case with abandoned partial paste

It is possible to abandon a paste while editing it on the web UI,
causing the database to have a state that the API was not expecting.
This filters out such pastes.

When we rewrite the paste.sr.ht frontend to work via GraphQL, we should
change the column to non-nullable and remove this where clause.
API: Handle null filenames properly

In the future we might want to normalize the database on empty strings
instead of null to represent this case.
API: update core-go
Add GraphQL riggings to Python module
API: Rename sha => hash
API: update go.*
API: Add note about leaking blobs on delete

We can address this later, it's not really a big deal.
Drop legacy webhooks

No one is using these in production.
API: Rig up mutation { delete }
API: Rig up mutation { update }
API: Rig up mutation { create }
API: Rig up file { contents }
API: Rig up File { sha }
API: Rig up paste { files }
API: Rig up query { paste }
API: Rig up Paste { user }
API: Rig up user { pastes }
API: Rig up query { pastes }
API: Rig up loaders, query { user }