Unify requests to wikibase with wbFetcher

[jmformenti]

Currently we have requests to Wikibase in different places within the code, for example: https://github.com/PhiloBiblon/philobiblon-ui/blob/master/frontend/pages/wiki/_page.vue#L30 https://github.com/PhiloBiblon/philobiblon-ui/blob/master/frontend/service/wikibase.service.js#L46 https://github.com/PhiloBiblon/philobiblon-ui/blob/master/frontend/service/wikibase.service.js#L172

The idea is use wbFetcher method as standard way to implement these request:

1. wbFetcher will receive a single input parameter: url
2. The method will check the url in a cache, if exists, the cached response will be returned otherwise it will perform the request.