Merge MutableAppContext into AppContext

There may have been a good reason for the difference at some point, or I was
still learning Rust. But now it's just &mut AppContext vs &AppContext.

crates/feedback/src/feedback.rs

@@ -6,7 +6,7 @@ pub mod submit_feedback_button;
 use std::sync::Arc;
 
 mod system_specs;
-use gpui::{actions, impl_actions, ClipboardItem, MutableAppContext, PromptLevel, ViewContext};
+use gpui::{actions, impl_actions, ClipboardItem, AppContext, PromptLevel, ViewContext};
 use serde::Deserialize;
 use system_specs::SystemSpecs;
 use workspace::{AppState, Workspace};
@@ -28,7 +28,7 @@ actions!(
     ]
 );
 
-pub fn init(app_state: Arc<AppState>, cx: &mut MutableAppContext) {
+pub fn init(app_state: Arc<AppState>, cx: &mut AppContext) {
     let system_specs = SystemSpecs::new(&cx);
     let system_specs_text = system_specs.to_string();
 

crates/feedback/src/feedback_editor.rs

@@ -11,8 +11,8 @@ use futures::AsyncReadExt;
 use gpui::{
     actions,
     elements::{ChildView, Flex, Label, ParentElement, Svg},
-    serde_json, AnyViewHandle, AppContext, Element, ElementBox, Entity, ModelHandle,
-    MutableAppContext, PromptLevel, RenderContext, Task, View, ViewContext, ViewHandle,
+    serde_json, AnyViewHandle, AppContext, Element, ElementBox, Entity, ModelHandle, PromptLevel,
+    RenderContext, Task, View, ViewContext, ViewHandle,
 };
 use isahc::Request;
 use language::Buffer;
@@ -35,7 +35,7 @@ const FEEDBACK_SUBMISSION_ERROR_TEXT: &str =
 
 actions!(feedback, [GiveFeedback, SubmitFeedback]);
 
-pub fn init(system_specs: SystemSpecs, app_state: Arc<AppState>, cx: &mut MutableAppContext) {
+pub fn init(system_specs: SystemSpecs, app_state: Arc<AppState>, cx: &mut AppContext) {
     cx.add_action({
         move |workspace: &mut Workspace, _: &GiveFeedback, cx: &mut ViewContext<Workspace>| {
             FeedbackEditor::deploy(system_specs.clone(), workspace, app_state.clone(), cx);