[Enhancement] Multi-repository bulk actions.

[RokeJulianLockhart]

Please add checkboxes to the repository list, so that I can

• [ ] clone (encompasses https://github.com/desktop/desktop/issues/3849#issue-290913917)
• [ ] pull, and
• [ ] refresh

multiple repositories simultaneously. Sometimes, I have to refresh 80 repositories, and with Gittyup, it is Hellish.