Add the ability to expand and collapse sections of the contacts panel

Also, allow joining projects using the keyboard.

crates/theme/src/theme.rs

@@ -249,6 +249,7 @@ pub struct ContactsPanel {
     pub contact_button_spacing: f32,
     pub disabled_contact_button: IconButton,
     pub tree_branch: Interactive<TreeBranch>,
+    pub section_icon_size: f32,
 }
 
 #[derive(Deserialize, Default, Clone, Copy)]