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/v3/views/login/Index.vue

@@ -104,6 +104,7 @@ const ERROR_MESSAGES = {
   'no-account-found': 'LOGIN.OAUTH.NO_ACCOUNT_FOUND',
   'business-account-only': 'LOGIN.OAUTH.BUSINESS_ACCOUNTS_ONLY',
 };
+import alertMixin from 'shared/mixins/alertMixin';
 
 export default {
   components: {
@@ -112,7 +113,7 @@ export default {
     Spinner,
     SubmitButton,
   },
-  mixins: [globalConfigMixin],
+  mixins: [globalConfigMixin, alertMixin],
   props: {
     ssoAuthToken: { type: String, default: '' },
     ssoAccountId: { type: String, default: '' },
@@ -186,15 +187,15 @@ export default {
         setTimeout(callback, 0);
       }
     },
-    showAlert(message) {
+    showAlertMessage(message) {
       // Reset loading, current selected agent
       this.loginApi.showLoading = false;
       this.loginApi.message = message;
-      bus.$emit('newToastMessage', this.loginApi.message);
+      this.showAlert(this.loginApi.message);
     },
     submitLogin() {
       if (this.$v.credentials.email.$invalid && !this.email) {
-        this.showAlert(this.$t('LOGIN.EMAIL.ERROR'));
+        this.showAlertMessage(this.$t('LOGIN.EMAIL.ERROR'));
         return;
       }