Link back to Matrix message for issues created with `!gitlab issue create ...`

[alexgleason]

When an issue is created (with !gitlab issue create ...) the bridge writes the issue content exactly as given with no additional information.

I suggest including a horizontal line (hr) and some text like "This issue was submitted through Matrix. View the message."

where 'View the message' is a riot.im link in the format of: https://riot.im/app/#/room/!roomID:matrix.org/$messageID:matrix.org

(it might be more "official" to use matrix.to links, but I don't like them. not ultimately my choice, though.)

This has a couple of advantages:

1. It's useful to know if an issue has created in a nonconventional way. The same would be true if you created an issue via email (for example). Bridges like this are hard to maintain because both sides of the bridge are independent. Some bugs are expected, but at least we can tell when the bridge is involved.
2. Linking back to the Riot message gives GitLab users the ability to jump right back into that chat conversation at any time. So when we decide we need a feature and have a whole conversation about it in the chat, we can access that chat log 6 months later when the feature is actually being implemented.

Thanks for your work on this!! It's been a great help to the communities I'm a part of. 😄