[Feature] (App): Perform bulk updates in repository dashboard

[dacharyc]

Is your feature request related to a problem? Please describe.
When I'm adding a lot of repositories, and the repositories have a lot of PRs, the main dashboard view can feel quite cramped when triaging hundreds of PRs.

Describe the solution you'd like
It would be nice if I could also perform bulk updates in the repository dashboards. Then I could work through the high-priority repositories first and possibly go back later and do the less urgent ones.

Describe alternatives you've considered
It might be nice to have a dedicated "Inbox" where I can view all PRs that I haven't triaged yet as a single list. It would give each card more room, and could be organized by repo similar to the other combined PRs views.

[dacharyc]

Although the request is to perform bulk updates in the repo dashboard, I think there is a place for this work to come along with the sidebar enhancements. I've gotten an offline request for the Incoming PRs "Inbox" dashboard, so I'll formalize that as a new feature request... and that brings along the question of whether to also have Assigned, Reviewing, and Mine options for the sidebar... so moving this to the "Sidebar customization" milestone.