Support multiple app updaters

[lberrymage]

Currently only one developer can submit updates for a given app, while in practice, team workflows may require multiple users to have this ability. Parcelo already supports this at a fundamental level, i.e., multiple updaters are supported by the database schema and REST API. However, there is no way to manage this access within the console itself.

The MVP for this feature consists of the following tasks:

• Support offering another user specified access permissions to an app one has sufficient permissions for in the backend
• Support this workflow in the frontend
• Support accepting such permission offers in the backend
• Support this workflow in the frontend
• Support revoking access permissions after they're accepted
• Support withdrawing access permission offers before they're accepted