Build on windows support [WIP, No Merge]

[Zexbe]

1. Conditionally support background, so fork does not block windows from compiling
2. Use mime_guess instead of magic crate.

Fix: #580

I am going to split this up into smaller pull requests

[jyn514]

The fork() stuff looks fine to me, we don't run on windows in prod and it will be very clear that it doesn't work if someone runs cratesfyi daemon.

I'm a little more concerned about the change to tree_mime since it hasn't been updated in a year. I also took a look about how we use mime types and it looks very fragile to me: we treat anything that doesn't start with text/ as binary (even application/json and similar). We should add tests to be sure the behavior is the same, at least for Rust, HTML, CSS, JS, markdown, and TOML.

[jyn514]

Looks good to me! I'll wait to merge until @pietroalbini gets a chance to look at it.

[jyn514]

---- web::rustdoc::test::settings_html stdout ----
the test failed: database error: ERROR: null value in column "mime" violates not-null constraint

Try adding a migration to src/db/migrate.rs, there's lots of examples in that file.