Start work on styling of follower avatars in facepiles

* Make follower avatars smaller than top-level avatars
* Make avatars in facepiles overlap
* Render an opaque background behind avatars in facepiles.

crates/theme/src/theme.rs

@@ -74,12 +74,13 @@ pub struct Titlebar {
     pub container: ContainerStyle,
     pub height: f32,
     pub title: TextStyle,
-    pub avatar_width: f32,
     pub avatar_margin: f32,
     pub avatar_ribbon: AvatarRibbon,
+    pub follower_avatar_overlap: f32,
     pub offline_icon: OfflineIcon,
-    pub avatar: ImageStyle,
-    pub inactive_avatar: ImageStyle,
+    pub avatar: AvatarStyle,
+    pub inactive_avatar: AvatarStyle,
+    pub follower_avatar: AvatarStyle,
     pub sign_in_prompt: Interactive<ContainedText>,
     pub outdated_warning: ContainedText,
     pub share_button: Interactive<ContainedText>,
@@ -88,6 +89,13 @@ pub struct Titlebar {
     pub toggle_contacts_badge: ContainerStyle,
 }
 
+#[derive(Clone, Deserialize, Default)]
+pub struct AvatarStyle {
+    #[serde(flatten)]
+    pub image: ImageStyle,
+    pub width: f32,
+}
+
 #[derive(Deserialize, Default)]
 pub struct ContactsPopover {
     #[serde(flatten)]