Increase issue label fetch limit

[mchisolm0]

Summary

Increase the limit of labels GraphQL can fetch to 20 from 3.

How did you test this change?

1. Ran go build
2. Started the extension with ./gh-dash
3. Navigate to an issue with more than 3 labels

Images/Videos

[dlvhdr]

@mchisolm0 looks good, just need to make sure the labels wrap nicely if the sidebar width is too narrow

[dlvhdr]

Looks good!