Starts work on #55.
This is similar to the previous work in #125. It's gated behind a new Cargo
feature, `user-plugins`. This time, the config gate is much smaller. The
`plugins` member of projects is still accessible when plugins aren't enabled,
but is always empty. Additionally, user plugins are only enabled if there's a
Lua state present in the snapshot context when the `SnapshotUserPlugins`
snapshot middleware runs. This not ever the case currently.
This code has very little possibility of rotting while we focus on other work,
since it'll be guaranteed to still compile and can be tested in isolation
without the feature being turned on.
* Refactor contributing_paths into contributing_sources, deleting project node sources
* Instead of changing contributing_paths, add instigating_source
* Remove InstanceMetadata::project_node
* Stop pushing project path to front of contributing_paths since it doesn't matter now
* Remove accidental UI change for path display
* Start actually computing AppliedPatchSet values
* Improve patch_apply documentation and flesh out applied patch code
* Add file link notes
* Stub out where tests for snapshot subsystem will go
* Create baseline tests
* Fix build failure by silencing Clippy
