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/MessagesView.vue

@@ -324,12 +324,12 @@ export default {
   },
 
   created() {
-    bus.$on(BUS_EVENTS.SCROLL_TO_MESSAGE, this.onScrollToMessage);
+    this.$emitter.on(BUS_EVENTS.SCROLL_TO_MESSAGE, this.onScrollToMessage);
     // when a new message comes in, we refetch the label suggestions
-    bus.$on(BUS_EVENTS.FETCH_LABEL_SUGGESTIONS, this.fetchSuggestions);
+    this.$emitter.on(BUS_EVENTS.FETCH_LABEL_SUGGESTIONS, this.fetchSuggestions);
     // when a message is sent we set the flag to true this hides the label suggestions,
     // until the chat is changed and the flag is reset in the watch for currentChat
-    bus.$on(BUS_EVENTS.MESSAGE_SENT, () => {
+    this.$emitter.on(BUS_EVENTS.MESSAGE_SENT, () => {
       this.messageSentSinceOpened = true;
     });
   },
@@ -396,7 +396,7 @@ export default {
       this.$store.dispatch('fetchAllAttachments', this.currentChat.id);
     },
     removeBusListeners() {
-      bus.$off(BUS_EVENTS.SCROLL_TO_MESSAGE, this.onScrollToMessage);
+      this.$emitter.off(BUS_EVENTS.SCROLL_TO_MESSAGE, this.onScrollToMessage);
     },
     onScrollToMessage({ messageId = '' } = {}) {
       this.$nextTick(() => {
@@ -514,7 +514,7 @@ export default {
       } else {
         this.hasUserScrolled = true;
       }
-      bus.$emit(BUS_EVENTS.ON_MESSAGE_LIST_SCROLL);
+      this.$emitter.emit(BUS_EVENTS.ON_MESSAGE_LIST_SCROLL);
       this.fetchPreviousMessages(e.target.scrollTop);
     },