Parse, highlight, link usernames in code

Description

I would like to have usernames, e.g., @tvierjahn, that are used in comments inside the code, being highlighted in the code view. Consequently, the files that they are in, should be added to some list "you are being mentioned in these files/projects" on my dashboard.

This is especially useful for TODO comments.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.