Recognise and persist prebuild definitions on template import

[SasSwart]

The first moment at which any component of Coder becomes aware of a prebuild is when a template is imported that defines that prebuilds. For anyone to be able to use prebuilds, we must therefore detect them when we import a template and persist them to the coder database.

[SasSwart]

The PR that closes this issue has been reviewed and approved:
coder/coder#16951

But cannot be merged until coder/coder#16891 is merged.

This issue blocked until then