[PR] Reduce archive templates in favor of index.php, fix pagination

All of our archive templates - `category.php`, `archive.php`, `tag.php`, and `index.php` have the exact same markup requirements. Rather than maintaining all of them, we can condense them into `index.php` and modify the `main` class name per view.

This also ensures we'll do a better job at handling some of the unknown archive views like year, month, author, custom taxonomies, etc...

If markup requirements change for a specific archive view, we can then split that off intentionally.