Save and reuse VM init scripts at the project level

Today, VM init scripts are stored only on the VM itself at creation time. There is no reusable saved script concept similar to registered SSH public keys or no normal UI path to select from previously saved init scripts when creating another VM

This makes repeated VM provisioning workflows awkward, since I have to repaste the same script for every new VM.

Proposed behavior:

• Add support for saving init scripts as reusable project level objects
• Allow selecting a saved init script during VM creation, similar to registered SSH public keys

We can keep UX similar to SSH Public Keys, e.g. "Manage Init Scripts" option on Settings page to add / edit / remove scripts.

I would be happy to take this on if the direction sounds right.