Versioning process migration: current to future

[thibaudcolas]

Part of #403. Follow-up to #445. We want to move from page-based versioning to block-based.

Tasks:

• Figure out whether / how we want to archive existing versioned content.
• Remove current versioning system from the site #452

Steps

1. Add new "version highlight" blocks/content to existing "Latest" pages, for the last 1-2 releases
2. Set up redirects system that sets a version picker to the target version and hides corresponding blocks for new versions.
   a. Simpler alternative: redirect all requests for unknown locales to english-latest
3. Delete/archive existing pages for old versions

[thibaudcolas]

This is now well under way via #453. There are a few things left to do:

• Clean up unused images
• Update documentation for new process
  • Guidelines on how we want to do versioning in the future - how many versions do we keep track of within pages (currently supported ones?), whether we attempt to version images or not.