Use Fork Awesome as the default icon font, and rework settings

In 16e3058, a FONT_AWESOME setting was introduced and used as such:

    "https://kit.fontawesome.com/{{ FONT_AWESOME }}.js"

Also in 16e3058, the default way to fetch Font Awesome was changed to
Javascript (before we used to load is as CSS).

The present commit does several changes:

- Revert the default back to CSS (instead of JS), as I think that for a
  font, there's no need for Javascript. A font is a static asset, so
  it's enough to load it as a static CSS file, both for privacy and
  security. Also, think about those who disable JS in their browser.
- Remove the `FONT_AWESOME` setting, as it's a bit too limiting to force
  users into `kit.fontawesome.com`, it doesn't leave much space for
  configuration.
- Instead, introduce two variables `FONT_AWESOME_CSS` and
  `FONT_AWESOME_JS`, so that users have more freedom to define how they
  want to load the Font Awesome.
- Use Fork Awesome as the default, if no setting is given by user.
  Meaning that we get back to the `fa` css classes (instead of `fas` or
  `fab`)

README.md

@@ -26,4 +26,11 @@ Additional settings:
 
 - `BIO` - short biography to display in the sidebar, eg. `Hello world`
 - `PROFILE_IMAGE` - image to display in the sidebar, eg. `avatar.png`
-- `FONT_AWESOME` - Fontawesome kit, eg `https://kit.fontawesome.com/{{ FONT_AWESOME }}.js`
+- `FONT_AWESOME_CSS` - URL to get Font Awesome as CSS
+- `FONT_AWESOME_JS` - URL to get Font Awesome as Javascript
+
+By default, the theme uses **Fork Awesome**, which is fetched from
+`cdn.jsdelivr.net`. Fork Awesome is a drop-in replacement for Font Awesome v4,
+but is not compatible with Font Awesome v5 and later, so if you want to use
+Font Awesome v5+, setting `FONT_AWESOME_CSS` is not enough, you will also need
+to modify the templates.