Allow link types to have custom icons

[GuySartorelli]

If link types had their own icons, it would add some quick visual distinction between links of different types, which will help content authors quickly understand what they're looking at.

Notes

Max had a PR where this was being implemented: #45 - should be able to pick some stuff from this

ACs

• There are different icons for different links types
• Documentation shows how to add custom icons to custom link subclasses

PR

• NEW Link type icon #170

[GuySartorelli]

@sabina-talipova please link to the PR(s) in the issue description

[sabina-talipova]

@sabina-talipova please link to the PR(s) in the issue description

Updated

[GuySartorelli]

PR merged