Add a "glossary" system for term management

[terotik]

• add a model for Term Categories (these group Terms, below)
• add a model for Term (has a reference to a Category, a name, a rich text (HTML) description, and a "show in lists" boolean field)
• add admin for the above
• make it possible to refer to Terms from within HTML content (#7)
• make it possible to add a Wagtail Block that lists all list-visible terms in a given Category ("definition list") (#37)