Remove cx from ThemeSettings (#38836)

Before this change the active theme and icon theme were retrofitted onto
the ThemeSettings.

Now they're in their own new global (GlobalTheme::theme(cx) and
GlobalTheme::icon_theme(cx))

This lets us remove cx from the settings traits, and tidy up a few other
things along the way.

Release Notes:

- N/A

crates/repl/src/jupyter_settings.rs

@@ -19,7 +19,7 @@ impl JupyterSettings {
 }
 
 impl Settings for JupyterSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let jupyter = content.editor.jupyter.clone().unwrap();
         Self {
             kernel_selections: jupyter.kernel_selections.unwrap_or_default(),

crates/repl/src/repl_settings.rs

@@ -1,4 +1,3 @@
-use gpui::App;
 use settings::Settings;
 
 /// Settings for configuring REPL display and behavior.
@@ -17,7 +16,7 @@ pub struct ReplSettings {
 }
 
 impl Settings for ReplSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let repl = content.repl.as_ref().unwrap();
 
         Self {