Bring rustup.js and installation markup into alignment with rustup.rs

[brson]

This primarily makes the install page offer the 64-bit version of rustup-init for Windows when appropriate, per rustup.rs.

Also

• Adjust the "unknown" and "default" cases to offer both 64-bit and 32-bit windows installers
• rustup.rs and rust-www use the exact same rustup.js
• add the "If you are a WSL user..." language to all non-English pages
• fix style on vi-VN

Fixes #1049

[rust-highfive]

r? @ashleygwilliams

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[brson]

This needs to be reviewed and merged with https://github.com/rust-lang-nursery/rustup.rs/pull/1437.

[brson]

cc @steveklabnik

[brson]

@Diggsey will you merge the rustup PR? @steveklabnik will you merge this one?