R1:ac19c558225c

Formalize "manual" buildables in Harbormaster

Summary:
Ref T1049. Generally, it's useful to separate test/trial/manual runs from production/automatic runs.

For example, you don't want to email a bunch of people that the build is broken just because you messed something up when writing a new build plan. You'd rather try it first, then promote it into production once you have some good runs.

Similarly, test runs generally should not affect the outside world, etc. Finally, some build steps (like "wait for…