Add options for shortening and dismissing messages referencing GitHub issues or pull requests

[christolis]

Problem

If too many issues or pull requests are referenced, the bot effectively drowns the channel where the conversation is held.

Proposed Solution

We should add a "Dismiss" button on the message that references GitHub issues or pull requests.

A "Shorten" or "Summarize" button would also prove beneficial if someone wants a one-line version of all the references, in a similar way git log --oneline works, which shows all relevant commits but only the commit message as well as some other metadata show up in only one line.

These buttons should obviously only be accessible by staff members and the authors of the original message.

Context

This behavior stems from the GitHubReference class. If someone sends #id of an issue (e.g. #207) in a specified channel, the bot replies with an embed that contains info on the issue/PR.

Media

Think about the members with a small screen, these embeds are taking all the space, not everybody likes that.

[Zabuzard]

(Not a good-first-issue without a detailed implementation guidance. Removed the label)