Added default avatars for groups and photo galleries

Based on the Circle's default avatar code with one notable change: We choose a avatar color based off of the room name versus randomly choosing a color. This prevents the issue where the avatar color changes each time it is rendered.

I think the initials doesn't work as it was intended, but I kept it as is for consistency. Later when display names are fixed when rendering the circle avatars, perhaps the same algorithm can be used.