Pull requests: let PR links open in my default browser again

[junxit]

Summary

• Clicking a PR title in the app used to open the PR on GitHub in my default browser
• Since the Sept 13 release ([filter_box] allow empty filters list (#7220) #7244) it opens the in-app pull request pane instead, so getting to GitHub now takes an extra click through the pane's GitHub icon
• I couldn't find a setting to change this. Settings → Links covers URLs, files, folders, ports and Pages, but not pull requests

Today
Every PR entry point (PR title, sidebar PR icon, "PR created" toast) opens the pane, with no way to go straight to the browser.

Proposal
Add a Pull requests row to Settings → Links with an on-click choice between the Superset pane (today's behavior) and the default browser. A modifier-click such as ⌘-click that opens the PR on GitHub would also work.