Add `upstream` remote from GitHub/GitLab

**Is your feature request related to a problem? Please describe.**

I'm always frustrated when I need to rebase against the parent and have no upsteam remote in my clone.

**Describe the solution you'd like**

In remotes menu press (a)dd (u)pstream to detect if the repo is a GitLab/GitHub fork and add `upstream` remote.

**Additional context**

Current shortcut for adding remote is (n)ew, which is not intuitive, because plain git uses `git remote add` with no (n) letters. I don't know if renaming or deprecating it is possible.

An alternative is to use (u) shortcut for upstream.