feat: add srcset for post image

[rise-erpelding]

Description

Implements responsive images on posts using srcset. src/php/views/partials/content-single.twig still exclusively uses the thumbnail (150x150) image.

Addresses #111

To Validate

1. Make sure all PR Checks have passed
2. Pull down this branch
3. Run npm start
4. Add a post with a featured image and navigate to it.
5. Open the Network tab in the inspector with a smaller screen width (around 300-400px), and reload the page. Note the name/size of the featured image that loads (it may be 300px wide/medium or 768px wide/medium_large), and slowly increase the size of the page, depending on your browser, you should see it load at least one other image size (1024px wide/large) before the browser window gets to 1000px wide. Different browsers seem to load different sized images and different widths as far as I could tell, but generally speaking smaller images are loaded for smaller screen widths.