Consider mapping undo to C-z and redo to C-y to be similar to keybindings from other software

**Is your feature request related to a problem? Please describe.**
It bothers me a little that undo is mapped to `z` while redo is mapped to `C-z`, because from years of dealing with other software I'm going to hit `C-z` by instinct when I want to undo something.

**Describe the solution you'd like**
I'd prefer if undo was `C-z` by default, while redo was `C-y` (there's a roughly even split between programs that use `C-y` for redo and those that use `C-S-z` for redo, but I've always preferred `C-y`). This would also require the current mapping of `C-y` (Copy pull request URL to clipboard) to be mapped to something else; `C-u` for URL is currently unused and could work.

**Describe alternatives you've considered**
I could, of course, remap these myself in `config.yml`, and probably will. But since the undo/redo feature is relatively new, it's probably not too late to change the default config, since not too many people will have gotten used to the current setup yet. So it's worth having a public discussion of whether people prefer the current keybindings, or whether more people would like `C-z` to mean "undo" and `C-y` to mean "redo".