Add a loading icon when generating sheets

[MoralCode]

if you add a BUNCH of label images it can take kind of a while to process, but almost always completes eventually. maybe some kind of loading animation to show people its doing something could help?

[MoralCode]

the project this is forked from (MoralCode/BingoGenerator) has this in the form of a gif, so we just need to pick an appropriate creative commons licensed gif to use. ideally something cute and related to PDF generation (like a little blob similar to the one from cut the rope eating some paper or something)