Add top-level API route for `changes`

[Mr0grog]

You can get the changes related to a page at:

/api/v0/pages/{page ID}/changes

This should also be available at the top level:

/api/v0/changes

This requires:

• Changes to config/routes.rb
• Make sure app/controllers/api/v0/changes_controller.rb still functions with this
• Add documentation to swagger.yaml

This is based on #239, which is directed at more speculative ideas that depend on this first.