Include issue/MR description when notifying about first @ mention

Description

I often create issues and want to make sure that specific people see them. Putting /cc with an @ mention in the issue description feels wrong since the person isn't really part of the issue description. But if I only CC them in a subsequent comment, then the notification email that goes out loses all the content of the issue description. Perhaps the first time someone is @ mentioned on an issue/MR, they should receive not only the content of the comment where they were mentioned, but the issue/MR description as well. Heck, perhaps they should even receive the entire thread, although that has obvious challenges.

Proposal

• When first notifying someone about being mentioned on an issue/MR, include the description.
• [Optional] Include content of entire discussion thread. If we do this, it might need to be in reverse chronological order.

Links / references

• Related: gitlab-ce#22093