Change templates to use snake_case instead of kabob-case

[thephm]

I realized a couple of things about my frontmatter field names, namely that using kabob-case style field names has two negative side effects:

1. when you double click on them in Obsidian or VS Code, it only selects kabob and not the entire field name
2. when I try to use them directly in Python using setattr() or getattr(), the dash - causes issues

So, for these two reasons I'm changing the templates and my code to use snake_case instead, i.e. using underscore character _ instead of dash character - to separate words in the field_names.