Create 2 filters: new issues and new pull requests in web UI

[ArcTanSusan]

Feature Request

Two new additional filters would help to filter out the notifications on a github repo: a filter for new issues and a filter for new pull requests. We currently have 2 filters: Unread and All at the top of the screen.

A filter for new issues would help contributors sort through which issues that are opened recently. A filter for new pull requests would help project committers/project leaders/maintainers find the PRs to help review someone else's code.