Inconsistent behavior of the mention notifications in JSM issues when batching is enabled vs disabled

Issue Summary

The logic that decides whether a mention notification should be sent from a JSM issue when a user does not have application access (no license) is different, depending if batched notifications are enabled or not.

Note: JSM refers to "Jira Service Management"

Environment

• Replicated with JSM 5.12.6, but we suspect that this behavior is replicable in any version from 5.0.0 (and potentially any version from 4.0.0)
• This issue does not occur with Jira Core and Jira Software projects, but only with JSM projects
  • In Jira Core and JSW projects, the mention notification will be sent to unlicensed users, whether notification batching is enabled or not
• The Feature Flag mentioned in the KB article Jira notifications (batched and non-batched) not sent anymore to users who don't have application access after Jira upgrade to 8.19.0+ has no impact on this issue, since it does not impact how mention notifications work

Steps to Reproduce

1. Setup a JSM application on any version from 5.0.0
2. Make sure that Batched Notifications are enabled in ⚙ > System > Batching email notifications
   1. To speed up the testing process, set batching to 2min
3. Create a JSM project
4. Create a new user, and don't grant this user application access
5. Make sure that this user is granted the browse projects permission in this project (via the Permission Scheme configuration)
6. Create a new JSM issue
7. Tag the user in this JSM issue
   
8. Wait for about 2min

Expected Results

The mentioned users should receive a notification saying that someone mentioned them in the ticket, as it's the case when Batched Notifications are disabled:

Actual Results

No notification is sent to the user.

Workaround

Grant the user application access (via a license)

Side note

External notification is sent when the ticket is created by the customer via the service desk.

Suggested fix for the bug

Make the behavior consistent across all types of projects, and regardless is the notification batching is enabled or not, by:

• either not sending the mention notification to unlicensed users
• or sending the mention notification to unlicensed users