Add context to structured logging messages

• Include repository metadata (if present) in each gRPC request context
• Replace our own log handler with github.com/grpc-ecosystem/go-grpc-middleware/logging/logrus
• Add repository metadata to Sentry tags

Closes #256 (closed)

This change ensures that log messages are consistently logged with certain request fields.

Initially, this will just be the repository path and storage. More fields may follow in future.

Additionally, sentry errors will include repository path and storage, as well as client information etc