Wikidata cache as a class

[marfox]

• Make a class for the cache so it's also easier to switch to a new backend if needed. See Validation criterion 3: biographical data #419 (comment)
• don't call public validation functions multiple times. See Validation criterion 3: biographical data #419 (comment)
• alternative: consider https://github.com/reclosedev/requests-cache/

Originally posted by @e-dorigatti in #419