chore: Replace eventBus with mitt.js [CW-3275] (#9539)

# Replace the deprecated `eventBus` with mitt.js

## Description

Since eventBus and it's respective methods are deprecated and removed
from all future releases of vue, this was blocking us from migrating.
This PR replaces eventBus with
[mitt](https://github.com/developit/mitt). I have created a wrapper
mitt.js to simulate the same old event names so it's backwards
compatible, without making a lot of changes.


Fixes # (issue)

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

1. Made sure all the places we're listening to bus events are working as
expected.
2. Respective specsf or the events from mitt.


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/components/widgets/conversation/ReplyBox.vue

@@ -596,12 +596,15 @@ export default {
     );
 
     this.fetchAndSetReplyTo();
-    bus.$on(BUS_EVENTS.TOGGLE_REPLY_TO_MESSAGE, this.fetchAndSetReplyTo);
+    this.$emitter.on(
+      BUS_EVENTS.TOGGLE_REPLY_TO_MESSAGE,
+      this.fetchAndSetReplyTo
+    );
 
     // A hacky fix to solve the drag and drop
     // Is showing on top of new conversation modal drag and drop
     // TODO need to find a better solution
-    bus.$on(
+    this.$emitter.on(
       BUS_EVENTS.NEW_CONVERSATION_MODAL,
       this.onNewConversationModalActive
     );
@@ -609,10 +612,13 @@ export default {
   destroyed() {
     document.removeEventListener('paste', this.onPaste);
     document.removeEventListener('keydown', this.handleKeyEvents);
-    bus.$off(BUS_EVENTS.TOGGLE_REPLY_TO_MESSAGE, this.fetchAndSetReplyTo);
+    this.$emitter.off(
+      BUS_EVENTS.TOGGLE_REPLY_TO_MESSAGE,
+      this.fetchAndSetReplyTo
+    );
   },
   beforeDestroy() {
-    bus.$off(
+    this.$emitter.off(
       BUS_EVENTS.NEW_CONVERSATION_MODAL,
       this.onNewConversationModalActive
     );
@@ -625,7 +631,7 @@ export default {
         const lines = title.split('\n');
         const nonEmptyLines = lines.filter(line => line.trim() !== '');
         const filteredMarkdown = nonEmptyLines.join(' ');
-        bus.$emit(
+        this.$emitter.emit(
           BUS_EVENTS.INSERT_INTO_RICH_EDITOR,
           `[${filteredMarkdown}](${url})`
         );
@@ -867,8 +873,8 @@ export default {
           'createPendingMessageAndSend',
           messagePayload
         );
-        bus.$emit(BUS_EVENTS.SCROLL_TO_MESSAGE);
-        bus.$emit(BUS_EVENTS.MESSAGE_SENT);
+        this.$emitter.emit(BUS_EVENTS.SCROLL_TO_MESSAGE);
+        this.$emitter.emit(BUS_EVENTS.MESSAGE_SENT);
         this.removeFromDraft();
         this.sendMessageAnalyticsData(messagePayload.private);
       } catch (error) {
@@ -1194,7 +1200,7 @@ export default {
     resetReplyToMessage() {
       const replyStorageKey = LOCAL_STORAGE_KEYS.MESSAGE_REPLY_TO;
       LocalStorage.deleteFromJsonStore(replyStorageKey, this.conversationId);
-      bus.$emit(BUS_EVENTS.TOGGLE_REPLY_TO_MESSAGE);
+      this.$emitter.emit(BUS_EVENTS.TOGGLE_REPLY_TO_MESSAGE);
     },
     onNewConversationModalActive(isActive) {
       // Issue is if the new conversation modal is open and we drag and drop the file