[nexus] Make project creation unwind safe, add tests

[smklein]

• Defines undo actions for project creation saga
• Adds test for the project creation saga

Part of #2052

[zephraph]

Suggested change

	let (_authz_project, project) =
	sagactx.lookup::<(authz::Project, db::model::Project)>("project")?;
	let (.., project) =
	sagactx.lookup::<(authz::Project, db::model::Project)>("project")?;

Would this work?

[smklein]

They're functionally the same; I was just being explicit about "what is the unused value" in this case since deserialization is particularly important on the lookup function, which is generic based on the type parameters.

[smklein]

Also, to be pedantic in the context of let binding:

1. let (.., foo) = ... means "destructure and bind one or more values to the anonymous "...", but only bind the last value to foo"
2. let (_, foo) = ... means "destructure and bind exactly one value to _, and bind the second value to foo"
3. let (_unused, foo) = ... means "destructure and bind exactly one value to _unused, and bind the second value to foo"

Technically in cases (1) and (2), since the bind is anonymous, it also goes out of scope, and drop is called immediately on the unnamed values. In contrast, in (3), drop is only called on _unused when it actually goes out of scope.

However, I don't think authz::Project has a drop implementation, hence why this is the same.

So it's not exactly the same, but it's basically the same - I just used this for readability, since the types of values being pulled out of the sagactx.lookup are important to get right, and caller-specified.

[plotnick]

Left a (non-blocking) security related question, but otherwise looks great, especially the tests; verify_clean_slate is awesome and I'll be stealing that idea in the future.