Add default user avatar

[coslu]

Pull Request Description

Creates a UserAvatar widget similar to the existing CommunityIcon and refactors user avatars to use that widget. This makes it so that users without an avatar show a default avatar with the first letter of the display name (if applicable) or the user name.

Also uses CommunityIcon inside CommunityHeader to ensure communities without an icon show the default icon instead of a blank circle.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Before / After

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[ajsosa]

Nice change 👍