Add ability to set global packages

[EmmaRamirez]

This PR:

• Allows for a very crude form of setting a global package

Some questions:

• Should it be a specific format? i.e. would json make sense?
• Should the user be allowed to set multiple globals?
• What's the most logical way to use globals? Right now, I just have them as defaults if the user doesn't enter anything. But to me, that doesn't feel very intuitive at all.

Sorry if this PR is a bit crude, I just wanted to actually get started with it. Obviously, it'd be awesome if I could suggestions on improving it!

[himynameisdave]

This is awesome, you did a great job!

Answers to above questions:

1. Yes, I think JSON makes the most sense as that's already what's expected when the user is uploading from a package.
2. I think for now setting one global should be okay, and later if we want we can add in support for many global packages. I could see the use case for wanting many global packages too, like maybe you'd want some for personal projects and some for work projects. For now let's stick to just one global package though, that way we can get this out faster and then iterate on it from there :)
3. I think this should be the default behaviour, that makes it easier to quickly add your usual default labels, especially if you use the tool on a lot of different projects and always use the same set of labels. But you are right about it not being intuitive, so we we need to think of a better way to let the user know about it 🤔

[Jameskmonger]

My only feedback is that it doesn't appear to be very clear that it's using the default package. Maybe it should be put as the default value into the text box?

[himynameisdave]

@EmmaRamirez are you still working on this? If not I don't mind picking up from where you left off...