Add live-preview when editing Markdown files online (BB-3466)
In the Overview tab my README.md can be displayed. Unfortunately I have no preview of this, so this might take me several commits to get it right. It would be great if you offered a preview, so that I can write my readme directly in the browser, or at least paste it there to see it. For the issue tracker you already provide such a preview (albeit it is not previewing Markdown, but another format).