Currently we are storing user information that is considered PII in ChatMessage and MemorySource.
We need to perform real-time look up using User IDs instead for privacy concerns.

This should also address the problem where the list of participants shows a GUID instead of the user's name and profile picture.

PRs that need to have this improvment:
#1345
#1307

### Tasks
- [ ] https://github.com/microsoft/semantic-kernel/pull/1899
- [ ] Remove username instances from store in webapi + do real time on chat history load in webapp
- [ ] Load user photos from Graph