Notification email subject lines containing emoji tags should be resolved to unicode 🚀

GitLab allows you to add emoji tags to issue subject lines, and these get resolved into unicode emojis (or fallback images depending on the platform) when the GFM is rendered on the issue page:

Example: gitlab-org/gitlab-ce!10865

However, notification emails sent out for these issues contain the emoji tag rather than the emoji itself:

Now that we are using native unicode emoji on GitLab, there's no reason we shouldn't support them in email subject lines as well, when possible.