Add "Autoplans" to Harbormaster

Summary:
Ref T8095. Two general problems:

- I want Harbormaster to own all lint and unit test results.
- I don't want users to have to configure anything for `arc` to keep working automatically.

These are in conflict because generic lint/unit test ownership in Harbormaster requires that build targets exist which we can attach build results to. However, we can't currently create build targets on demand: Harbormaster assumes it is responsible for creating targets, then running code or making third-party service calls to actually run the builds.

I considered two broad approaches to let `arc` push results into Harbormaster without requiring administrators to configure some kind of "arc results" build plan:

# Add magic target PHIDs like `PHID-MAGIC-this-is-really-arc-unit`.
# Add new code to build real targets with real PHIDs.

(1) is probably a bit less work to get off the ground, but I think it's worse overall and very likely to create more problems in the long run. I particularly worry that it will lead to a small amount of special casing in a very large number of places, which seems more fragile.

(2) is more work upfront but I think does a better job of putting all the special casing in one place that we can, e.g., more reasonably unit test, and letting the rest of the code rarely/never care about this case since it's just dealing with normal plans/steps/targets as far as it can tell.

This diff introduces "autoplans", which are source templates for plans/steps. This let us "push" these targets into Harbormaster. Hypthetically, any process "like" arc can use autoplans to upload test/lint/etc results. In practice, probably only `arc` will ever use this, but I think it's still quite a bit cleaner than the alternative despite all the generality.

Workflow is basically:

- `arc` creates a diff.
- `arc` calls `harbormaster.queryautotargets`, passing the diff PHID and saying "I have some lint and unit results I want to stick on this thing".
- Harbormaster builds the plan, steps, and targets (if any of them don't already exist), and hands back the target PHIDs so `arc` has a completely standard-looking place to put results.
- `arc` uploads the test results to the right targets, as though Harbormaster had asked it to run unit/lint in the first place.

(This doesn't actually do any of that yet, just sets things up.)

I'll maybe doc turn that ^^^^^^ into a doc for posterity since I think it's hard to guess what an "autotarget" is, but I'm going to grab some lunch first.

Test Plan:
- Added unit tests to make sure we can build these things properly.
- Used `harbormaster.queryautotargets` to build autotargets for a bunch of diffs.
- Verified targets come up in "waiting for message" state.
- Verified plans and steps are not editable.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: hach-que, epriestley

Maniphest Tasks: T8095

Differential Revision: https://secure.phabricator.com/D13345