Refactor wikibase.store - introduce AbstractedRepoApi

* Remove the useless wikibase.EntityStore.js
* Create a new module wikibase.RepoApi which holds
  basic API functions (useful for the client
  as we have way less dependencies)
* Create wikibase.AbstractedRepoApi which extends
  the above with abstraction features (useful for
  the repo)
* Added missing dependency for wb.utilities
  while I was on it

Change-Id: If0f4ba3a3b4ff28330860ab308e71c8afaca6a6b
Merge "Refactor wikibase.store - introduce AbstractedRepoApi"