Wrong avatar in 1-to-1 conversations when using multiple users from same instance

## Steps to reproduce
1. Log in with User A in NC instance A
2. Log in with User B in NC instance A
3. Create a 1-to-1 conversation between User A and User B with User A

### Expected behaviour
User A sees User B avatar in their 1-to-1 conversation
User B sees User A avatar in their 1-to-1 conversation

### Actual behaviour
User A sees User B avatar in their 1-to-1 conversation
User B sees its own avatar in the 1-to-1 conversation with User A (since the same image cache is being used for every user in the app)

### Possible solutions

- Create different image downloaders for every user in the app
- Do not cache 1-to-1 conversations' avatars (not ideal)

_This might be considered an edge case since it's not common to have multiple users from the same instance in the app which have 1-to-1 conversations between them._