Link type plugins

[bartfeenstra]

• Allow Python packages to provide link types
• Add links to the project configuration, such as for social media, contact details, etc
• For Cotton Candy, display known link types with brand icons. Do this for entities (at the top of their pages), and for the project (in each page's footer)
• Refactor the Wikipedia extension to use this new API

Blocked by

• Create a new Plugin API #980