R1:0ac1be709434

Implemented support for build logs

Summary:
Depends on D7519.

This implements support for build logs in Harbormaster. This includes support for appending to a log from the "Run Remote Command" build step.

It also adds the ability to cancel builds.

Currently the build view page doesn't update the logs live; I'm sure this can be achieved with Javelin, but I don't have enough experience with Javelin to actually make it poll from updates to content in the background.

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