Initialization logic needed for serve, build, and upload is now
much more clear than it was when these functions were written.
This commit refactors all of them to use a new common_setup
module for all of their initialization that's the same.
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.
