cloudinary image replacements in format_post() are too greedy

[nottrobin]

This regex:

https://github.com/canonical-websites/blog.ubuntu.com/blob/2062ed1fa8e55e56142707c02040d49a32b54564/helpers.py#L124

Is too greedy. It results in any src="..." following an img being replaced, whether or not it's actually a property of the same element. It can easily catch srcs in code snippets etc., and ones that don't actually belong to <img> elements.

This can be quite frustrating for people trying to publish blog posts with code snippets.

[nottrobin]

We should probably use beautifulsoup rather than regex