Add a minimal image proxy

[kevinmcconnell]

To provide some safety when linking to user-supplied external images, we can provide a simple image proxy handler. Images accessed through this proxy will only be served if they meet the following criteria:

• Appear to be valid image files
• Are in a permitted format: GIF, JPEG, PNG or WebP
• Do not have an excessive width or height (5000 pixels max, by default)

To serve an image through this proxy, its URL should be passed to the handler's path as a src query param. The path is supplied to the application in the IMAGE_PROXY_PATH environment variable.

We'll also provide a helper method to make forming the proxy links easier:

Thruster.image_proxy_path('https://example.com/image.jpg')

This will result in a URL something like the following, which will route the image request through Thruster's proxy handler:

/_t/image?src=https%3A%2F%2Fexample.com%2Fimage.jpg

[palkan]

We'll also provide a helper method to make forming the proxy links easier:

I would suggest integrating with Active Storage instead (so no application changes would be required, just a configuration one); see, for example, https://github.com/imgproxy/imgproxy-rails