Shortnames for `Tag`s or strings

[joepio]

We use shortnames, but @AlexMikhalev wanted to import Articles from his markdown project and he used strings with spaces. We should consider using a string type.

Considerations:

• Most tags in systems are in fact short, dashed, URL-safe
• But definitely not all, including Github's issues for example

To edit:

• Ontology
• Tag Input (also fix error handling, unless it's no longer necessary)
• Pre-fill