Adjustments to allow monthly releases (Or "Releases") to be downloaded.

[MelissaAutumn]

• Add a DEFAULT_RELEASE_VERSION and use that for any version defaults.
• Some pre-built metadata for javascript.
• Add a warning about Thunderbird Releases.

Fixes #559

I'm not sure why I wasn't using a pre-built javascript metadata script tag, but now I am. Other random script tags we have will slowly migrate to that file.

Here I've split release into the in-testing monthly releases, and esr which will server...esr builds. I've also added a default setting so we can flip it to release in the future with hopefully minimal issues.

Screenshots