Don't render images in user profile timeline

Including images as part of the user profile timeline (i.e. https://gitlab.com/mikegreiling) really disrupts the flow of the page. IMO the User commented on merge request !12345 lines should just contain a single preview line of text.

Proposal

Remove images from profile timelines and show the first line of text instead.

Slack conversation: https://gitlab.slack.com/archives/C03MSG8B7/p1498148390648918

/cc @tauriedavis