Implements about:blank in an about_loader

1. First commit removes a redundant `proc` being created, as it wasn't closing over any variables. Instead, `*_loader::factory` can be called directly, *that* can spawn a task or what-have-you.
2. Without a dedicated "shipped content" location, I wasn't sure where to put an `about_blank.html` file. The `FIXME` in the same module seems to imply that it should not be in `src/test/html/about_blank.html`. Besides that, `about:blank` seems to be loaded quite often (many iframes seem to point to it), that it seemed worth skipping the IO overhead, and just keep the few bytes as a static.

Fixes #3012