Add "Bugs" tab #2749

<img width="454" alt="" src="https://user-images.githubusercontent.com/1402241/73672788-16f51300-46e0-11ea-862b-c1ea5a584aab.png">

It would strictly link to the bug label, which is the most common label name. The "Pinned issues" section would be hidden like it's hidden in the PR tab.

If no issues are found under the label, the tab wouldn't be added.

Useful or annoying? I feel that the current "Issues" tab hides bugs that need to be fixed by merging them with feature requests and all kinds of other noise.