Images should be hided or be replaced by their thumbnails in events feed

Description

Images in comments of issues may take too much height in the events feed.

Here's a test case:

This is the first line of a comment, this image: ![](image.png) will take all people's attention lol

Proposal

They should be hided or be replaced by their thumbnails.

Links / references

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/events/event/_note.html.haml#L10
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/helpers/markup_helper.rb#L69