Open at first unread comment

This is a patch to add a method called `-mostRelevantUrl` to the `PullRequest` entity. It currently checks for unread comments, and returns the `webUrl` of the first unread comment. Otherwise it returns the normal `webUrl` of the pull request.

I've utilised the method to open the first unread comment when the user clicks on a PR with one or more unread comments. This is a feature I've been wanting in the app for some time, as have a number of developers I work with. I'm happy to rework it to make it an optional behaviour, if you folks would prefer it.