communities: enable topics metadata

from @Jarvie : https://peakd.com/hive-102930/@jarvie/history-of-blockchain-communities-and-what-is-next

"We (peakd.com) would really love to be able to classify each community by theme/topic so that we can present new (and old) users with communities related to topics they tell us are of interest to them. So it would be a very good time to allow the community to identify X number of topics their community is related to. Perhaps 3 or 5. The number could be debated. It can be an open system where a community could put whatever they want. BUT perhaps a front end could help the communities to choose from a list of common selections. Like ARTS/SPORTS/BLOCKCHAIN etc. This would also be helpful when helping to sort communities as well. But mostly helpful to help present a new user with a list of Topics, Communities and maybe even Users that relate to subjects they like."

Basically this field already exists in the schema in two ways, not sure what @roadscape inteded those to be since they are different: category and primary_tag

I think that a single topic won't be enough, so we would store it as jsonb and use the updateProps action to update the topics. Then we would introduce a new api to fetch communities by topic (search for "sport" and get all communities that match).