Fixes#1967
Update the way responsive images are used to only use one srcset, and
add corresponding `sizes` attribute to the relevant layouts and
templates.
I did not go through and add `sizes` to all the example posts/projects.
When `sizes` is not specified by the user, the `figure.html` template
defaults to 95% the width of the viewport which should work fine for
most cases; users can optimize further if they wish by feeding `sizes`
into the template.
Additionally:
- Enabled support for .gif to .webp compression
- fix error in jekyll-imagemagick config where all images were resized
to be 800px or less. (for example img-1400.webp was actually only 800px
wide, etc.)
- added note about making sure imagemagick is installed before enabling
it in responsive images section of `_config.yml`
fb294e3348