Copy existing Diffs Vuex store to legacy diffs Pinia store

What does this MR do and why?

This is a first step towards Pinia migration for Diffs and Notes app. In this MR add Pinia and copy the existing Vuex store to migrate it in the next steps.

No visual change expected, no logic is migrated to Pinia here except for state declaration. Code is copied AS IS. Tests are disabled because they are also not migrated yet. getters and getters_versions_dropdowns are joined into a single module.