Multipart upload for actions and their attachment

We want to support actions with "large" bodies (e.g. zip file of JS dependencies, large jar files). Currently, action bodies are always uploaded as part of a JSON object; either the source file, or a base64-encoding of a binary. The JSON object also contains metadata for the action.

The multipart upload would accept the metadata and the action body separately. The action body could be streamed to the DB and would not have to be kept in memory, allowing us to raise size limits.