[Feat]: Web-based GPG pin entry for signing git commits

## What is your suggestion?
When signing git commits with GPG, where the GPG key exists on the server, display a prompt that allows for entry of the GPG pin.

## Why do you want this feature?
If you currently try to commit from the UI when using GPG signing, you get an error:

```
error: gpg failed to sign the data
fatal: failed to write commit object
```

...but if you use the terminal, it works fine:
```
git commit -m "My Commit Message"
```

This requires me to open the terminal every time I want to commit. I would prefer to use the nice UI.

## Are there any workarounds to get this functionality today?
Not that I know of. I could just be doing something entirely wrong.

## Are you interested in submitting a PR for this?
I wouldn't really know where to begin here, unfortunately, and lack the time at the moment to figure it out.