Change how CDN_HOST is passed down to make assets build reproducible

[ClearlyClaire]

Fixes #14379

This would enable us distributing the assets on release, sparing admins the need to recompile them (unless they want to make custom modifications, of course).

To achieve this, the CDN_HOST isn't used by webpacker/wepack in anyway anymore, instead:

• the public_path setting is passed to the env plugin (it can be different on custom configs or across test/development environments, but it's always the same value in default production builds regardless of CDN_HOST
• CDN_HOST, if defined, is passed by Ruby in a meta HTML tag, which is read to set both the assetHost constant used by Mastodon itself, and the __webpack_public_path__ variable used by webpack to override publicPath.

[ClearlyClaire]

This is actually not enough, as CDN_HOST is used for webpack's publicPath.
A wrapper around the entry point would probably be needed, as described in https://webpack.js.org/guides/public-path/#on-the-fly

EDIT: See also rails/webpacker#2269

[ClearlyClaire]

A few notes:

• This relies on the script being able to look up a meta tag in the DOM the moment is executed. I believe that is fine because the meta tag is guaranteed to be above the script in the DOM.
• I have not checked if the mailer was affected.