add 'jump to issue' button

[GabrielRMuller]

Related to #789

This is how the button looks, it only appears when the repository has issues.

Although the feature is ready and tested, I do think it could have some improvements on the selection of issues, so it doesn't just randomly selects one. Also on the feature request page, it's mentioned the addition of a rate limit, which I think it's a good idea, but I'm not quite sure the best place of adding this feature.

I'm open to any suggestions and I'll try to make the changes as soon as I can. Thanks!