Added support for Isso comments

[cmilanf]

Isso is an open source commenting server similar to Disqus that works well with low to medium traffic websites. It can be self-hosted. This PR adds support for it in Mapache without disturbing existing Disqus one. Disqus will still take precedence if the short name is present; if not, it will look for comments_isso_fqdn to enable the support.

[cmilanf]

I am closing this PR since I made further changes in the fork beyond the Isso comments support, being the most important ones:

1. #69b7bb0 Support for @page.show_title_and_feature_image in page.hbs. This breaks compatibility with Ghost versions prior to 5.54.1.
2. #b269113 Add support for Ghost Custom Fonts, as well as added Atkinson Hyperlegible font as default and JetBrains Mono font for code blocks. These added fonts are just based on my personal preferences and there is no particular reason behind the choice.

If any of these changes are useful for the upstream, feel free to take a look at them and ask any questions. I wanted also to thank you for the excellent work you made with this theme, I really like it.