Add /downloads/latest/prerelease redirect

[hugovk]

Description

One task for the release manager (👋) for every pre-release (and the .0 final) is to manually edit https://www.python.org/download/pre-releases/ to point to the new one.

Instead, let's create /downloads/latest/prerelease to redirect to the latest pre-release available.

For example:

• downloads/latest/prerelease -> /downloads/release/python-3150a2/

In the ~one week period when 3.x.0 has just been released and 3.x+1.a1 is not out yet:

• /downloads/latest/prerelease -> /downloads/

This PR also updates the download-banner and downloads-active-releases boxes in the test data to match production, for easier testing, and then to use the new redirects in these places:

After merge, I'll update:

• the boxes in prod to use the new redirect
• the old static page https://www.python.org/download/pre-releases/ to use the new redirect
• PEP 101 to remove "Update the “Current pre-release testing versions” page": peps#4717
• python/release-tools to remove modify_the_prereleases_page(): release-tools#308

[JacobCoffee]

needs rebase @hugovk :)

[hugovk]

Updated!