Fix server generation of 404 pages (#3103)
Any `404.js` page would inject a `[...]` path into the server generation. The problem comes from the `pathname_for_path` function, which will extract the extension from that path. In this case, it leads to a basename of `[...` and an extension of `]`, breaking the `regular_expression_for_path` dynamic route parser.
Unfortunately, our integration tests timeout when I try to write a basic test case for this, so it's not possible to test at the moment. I've manually verified.
Fixes WEB-214
