Add "Reference: " copy-pastable reference when viewing an issue

[neonn]

When viewing an issue in GitLab, you can quickly copy a reference to the issue being viewed.

This is useful for comments in issues, as it is a convenient way of referencing the issue in other issues in other Markdown inputs, or wherever shorthand issue references can be used. Below is an example issue using GitLab and a public issue. Note the "References: ..." string and icon in the bottom-right of the image, showing the reference.

The above example is from the Freedesktop GitLab instance.

The format of this string is <project-name>/<repository-name>#<repository-issue-index>, where:

• project-name: Name of the project (e.g. go-gitea)
• repository-name: Name of the repository (e.g. gitea)
• repository-issue-index: Index number of the repository (e.g. 1)

For example, a reference to a project might look like: go-gitea/gitea#2

Clicking the "Copy reference" button copies the reference to the issue to the user's clipboard.

I have discussed this a little with @Gusted on 2022/04/30 in Discord, in #develop. Thank you for the Go code! 😃

I will follow up with a PR in a moment.