🏃 Validate site action urls later and infer static

[fwkoch]

Site actions may be either static files, relative urls, or external urls. Previously, if static: true was specified and a file did not exist, the entire build failed. Also, if a local static file was specified but static: true was not, validation failed.

Now, action validation is a bit less strict and a bit more helpful:

• If the action points to an existing local file, static: true is inferred
• If the action says static: true but the file does not exist, it still logs an error, but the build continues
• If the action does not look like a URL or a local file, it warns, but the build continues.

[fwkoch]

The changes here became useful when working on this feature: #1049 - so everything is combined over there, and I'm closing this PR in favor of that one.