fix(BrowserStorage) - clear BrowserStorage when different user logged in

[Antreesy]

☑️ Resolves

• If another user is logged in, we should check, whether it's uid matches with last cached uid.
  • if it matches, we could safely restore and use data from storage;
  • if not, we wipe everything related to the last user and containing possibly sensitive data;

🖼️ Screenshots

No visual changes

🚧 Tasks

• Add encryption?
• Another possible checks if uid is not enough?

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not required
• 📘 API documentation in docs/ has been updated or is not required
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
• 🔖 Capability is added or not needed

[Antreesy]

In fix-up:

• don't save uid for non-autorized users: null === null on checking for guests, all preferences are keeped
• when there's a mismatch, clear only conversations (as it's the only part of sensitive data)
• TODO:
  • unify sensitive data somehow

[nickvergessen]

/backport to stable27