Add support for existing type categories

[ezwelty]

The four supported type categories reflect four different user communities: forager, freegan, grafter, honeybee. Support for these are needed by falling-fruit/falling-fruit-web#246 to handle historic URLs. However, they could also be seen as built-in type lists (see #9). The simplest solution would be to return categories of each type from GET types.

However, for performance (#2), it might make sense to pre-compute these type filters as indices on the locations table. This would avoid the need for filtering locations by a long list of type ids dynamically. This would instead require a new query parameter in GET locations (e.g. categories).

[ezwelty]

Types are now returned with their categories, which allows the most basic level of category support. Closing this issue untile additional functionality or performance is needed.