Implement cache for tagcloud
The tags for the tagcloud do not have to be re-queried on every use, instead a cache-mechanism should be implemented.
Note: This goes only for the overall cloud though since collection clouds are generated based on search results.
(from redmine: issue id 25, created on 2018-01-22tgradl, closed on 2018-01-22)
- relates #61 (closed)