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/store/modules/specs/conversations/mutations.spec.js

@@ -1,6 +1,16 @@
 import types from '../../../mutation-types';
 import { mutations } from '../../conversations';
 
+jest.mock('shared/helpers/mitt', () => ({
+  emitter: {
+    emit: jest.fn(),
+    on: jest.fn(),
+    off: jest.fn(),
+  },
+}));
+
+import { emitter } from 'shared/helpers/mitt';
+
 describe('#mutations', () => {
   describe('#EMPTY_ALL_CONVERSATION', () => {
     it('empty conversations', () => {
@@ -130,7 +140,7 @@ describe('#mutations', () => {
           timestamp: 1602256198,
         },
       ]);
-      expect(global.bus.$emit).not.toHaveBeenCalled();
+      expect(emitter.emit).not.toHaveBeenCalled();
     });
 
     it('add message to the conversation and emit scrollToMessage if it does not exist in the store', () => {
@@ -158,7 +168,7 @@ describe('#mutations', () => {
           timestamp: 1602256198,
         },
       ]);
-      expect(global.bus.$emit).toHaveBeenCalledWith('SCROLL_TO_MESSAGE');
+      expect(emitter.emit).toHaveBeenCalledWith('SCROLL_TO_MESSAGE');
     });
 
     it('update message if it exist in the store', () => {
@@ -195,7 +205,7 @@ describe('#mutations', () => {
           ],
         },
       ]);
-      expect(global.bus.$emit).not.toHaveBeenCalled();
+      expect(emitter.emit).not.toHaveBeenCalled();
     });
   });