Build: store build job in BuildCommand

[agjohnson]

This is the backend change to our builders to support storing the build job on BuildCommand.

See readthedocs/ext-theme#171 for more background on implementation. Note that the build detail view cannot currently use APIv3 because of missing models, so this would be a good chance to finally remove that and take a step towards deprecating APIv2.

• Refs Build: add more output details to the build detail page ext-theme#171
• Requires API v3: BuildCommands and missing fields to build v3 API #11179