Hugo v0.63 introduced breaking changes such as breaking templates that don't immediately begin with a `define` (even if the first line is just a code comment): https://gohugo.io/news/0.63.0-relnotes/
Adds compatibility with Hugo v0.63, fixing author pages rendering blank.
* check if the Hugo generated author profile URL is valid
* works around the Hugo bug, preventing invalid links
* rename `author` folder to `authors`
* no need to GetPage authors/x and then GetPage author/x to check if
user account was created
* can just rename the folder and use `if .File` instead
* more aligned with how Hugo's taxonomy system was designed
* refactor all author code
BREAKING CHANGE: rename `author` folder to `authors`
Fix#1064