We just copy the existing contents of `EditAvatar` and paste into `EditUserAvatar` and `EditThreadAvatar` and update imports.
In subsequent diffs we will:
1. Move `UserAvatar` and `ThreadAvatar` "into" `EditUserAvatar` and `EditThreadAvatar` respectively instead of passing in `React.Node` prop.
2. Pull out common functionality to hooks
3. Introduce user/avatar-specific functionality
4. **Once everything is working**, consider introducing some sort of `BaseEditAvatar` with some of the common styling/layout stuff (eg pencil icon). Defer this until the end once we have everything working.