Monthly automatic releases via GitHub Actions

[TheLastZombie]

This PR tracks the automatic generation and publishing of monthly releases of all versions of the script, including:

• [x] The userscript on GitHub (push build, create tag),
• [ ] the userscript on OpenUserJS (send webhook),
• [ ] the extension on AMO and Opera add-ons,
• [x] the website hosted on GitHub Pages and
• [ ] the Reddit bot (pull and restart).

The following repository secrets need to be set up:

• [ ] MAXURL_PEM (contents of the maxurl.pem file)
• [ ] API_KEY (JWT issuer, see: Manage API Keys)
• [ ] API_SECRET (JWT secret, see: Manage API Keys)

This entire workflow also hasn't been tested yet.

Depending on the eventual implementation, this could fix #506, fix #1044 and fix #1048.

[Luke-L]

so does this mean the extensions/addons will be updated with new supported sites automatically every month?

[TheLastZombie]

That's the plan, yes.

[TheLastZombie]

I'm closing this PR now since updates have resumed and I haven't been working on it for a while now.

[qsniyg]

Thank you for your work on this. I've always been a little queasy about automatic releases (especially in light of the whole xz situation), so instead I've been working on automating the build process as much as possible so it isn't as much of a pain :)