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/client/src/client.rs

@@ -101,7 +101,7 @@ pub struct ClientSettings {
 }
 
 impl Settings for ClientSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         if let Some(server_url) = &*ZED_SERVER_URL {
             return Self {
                 server_url: server_url.clone(),
@@ -133,7 +133,7 @@ impl ProxySettings {
 }
 
 impl Settings for ProxySettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         Self {
             proxy: content.proxy.clone(),
         }
@@ -519,7 +519,7 @@ pub struct TelemetrySettings {
 }
 
 impl settings::Settings for TelemetrySettings {
-    fn from_settings(content: &SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &SettingsContent) -> Self {
         Self {
             diagnostics: content.telemetry.as_ref().unwrap().diagnostics.unwrap(),
             metrics: content.telemetry.as_ref().unwrap().metrics.unwrap(),