Split the icon-font mixin into separate mixins, functions and variable files
Fix web-font include to use the correct filename
Rename icon-address() to icon-content()
Add a default $icon-list variable
Move the icon-replace() include to the top of the icon-base mixin

The reason is so that we are following our ordinary standard of having properties that effect the parent go first, following by properties that effect the parent's children.
Rename instances of `icon` to `icon-font`
Rename the _icon-content.scss to _icon-font-content.scss
Rename variables to be prefixed with $icon-font, and remove needless vars
Fix variables, some of which were renamed
Identify icon-font() mixin's dependencies
Remove old, obsolete comment
Move the default settings for icon-font into the correct folder
Merge branch 'update-icon-utilities' of github.com:mobify/spline into update-icon-utilities
Move icon font settings to the settings folder
Add tests for icon font
Use hide text mixin instead of manually declaring
Add a declared width to replaced icons so inline elements work automatically
Replace unnecessary headings with more informative comments

Also rearrange the settings variables in a slightly more logical order.
Remove unneeded heading
Remove the replace functionality from @mixin icon-font()