Curate category contributions

Lots of pull request have been raised to make our category directory better, and hence the search better for the user who easily finds buttons to click for searches like "restaurants", "crêpes", "nursery" (well, in French for now until #574).

We need to curate the numerous contributor proposals, add icons (and create them if needed, often).

⌚️2 days distributed along the year

Also, instrument the categories better by including "invisible" categories that won't be easily found in the directory but will be proposed on typed search.

See #721 where the discussion goes on.

⌚️ 1 day