Add "Notify on my actions" option for Slack notifications to individual users

Using the @user option for a Slack subscription, users can get direct messages from the Confluence app in Slack when spaces/pages have actions that trigger a notification. In the standard notification sense, this would be similar to a user selecting "Watch" on a page or space.

Confluence has a per-user setting for email notifications (toggled off by default) titled "Notify on my actions". This prevents a user's own changes from triggering an email notification to them.

Given that some users may try to use Slack subscriptions to replace email notifications, it would be helpful if the app had a similar setting. An example may be a user subscribing themselves to Slack notifications for their own personal space. The user would desire to receive notifications (via Slack) of changes from users other than themselves. Currently, they receive notifications of all changes, including their own.