[Notifications] Send out a notification on @mentions in Issue Activity tab Comments

### Problem Statement

Filing a feature request on behalf of a user in this [zendesk ticket](https://sentry.zendesk.com/agent/tickets/147493)

Our Issues Activity section allows for tagging users and teams. We should also send a notification when a user or team has been tagged in an issue.

Right now it seems we only send out a workflow notification if a user is subscribed to the issue (by default). At which point, they will receive a notification for _every_ comment, regardless of whether they were tagged or not. Tagging them seems to provide no extra functionality in terms of notifications.

### Solution Brainstorm

Send a new type of notification for being tagged. If a user has "Only Subscribed Issues" set in their notification settings, they should probably still be notified when tagged on an issue they are not subscribed to.

Or at least tagging a user should Subscribe that user to the issue if the user has "Only Subscribed Issues" in their settings

### Product Area

Issues

Start by tracing the Issues Activity comments flow and the existing workflow notifications for subscribed issues. Clarify whether tagging should notify users outside their subscriptions or subscribe them automatically, including team mentions. Done means the agreed behavior is implemented and verified for tagged users and teams without sending unrelated comment notifications.

Written by the indexing model from the issue text.