Icon support in Gitlab Flavored Markdown

Problem 🔴

We currently do not support embedding icons in GitLab Flavored Markdown.

Why do we need to solve this problem?

As part of the Error Tracking epic, we will soon be able (!20666 (merged)) to create an issue through the Sentry Error detail page. As per the design specs:

there's an external-link icon next to the Sentry Event link.

How can this issue be solved?

There are a few ways to go about this:

1. Add support for any type of icon to GFM
✅ Any type of icon can be showed, which would expand the functionality of GFM
✅ Icons will only show up if specified (see below)
⛔ Difficult to implement
⛔ We don't know if anyone wants this, so it may be overkill to do all of this because of one issue

2. Do a CSS solution that targets markdown with an after:: pseudo element that targets all links.
✅ Quick and easy solution
⛔ It would mean that every single link has the external icon next to it, which is a big design decision

3. Since we already have emoji support, perhaps we can create an external link emoji.
✅ Quick and easy
⛔ I don't know how to make emojis, so I'm not sure exactly what the con is 😬