Consider styling links and GFM references in user profile events and todos

When you visit a user's profile page (e.g. here), and an activity event contains a reference to something we style in GitLab Flavored Markdown (e.g. labels, usernames, issues, MRs, etc), we're not styling it as we normally do. All links are styled with .event-item a { color: #333; } (gray, same color as plain text ), so it seems that this is intentional.

But this practice is poorly suited for some gfm content items, as they still inherit some styles from gfm (e.g padding and margins), and end up with weird spacing. And it isn't clear why they have weird spacing, given that they're colored black like other plain text. I think this qualifies this issue as a gitlab-ce~2278648

Graying out gfm content also seems inadequate given that we're setting tooltips on these items. It seems to me bad UX to have clickable, tooltip'ed links that look exactly the same as plain text.

We already style code blocks on this page.