[Characters] Allow Export of Multiple Characters

Currently we can export individual characters as a PDF which is a great feature to have (lets us print them, or just back them up).

However, to do this for a large number of characters is very time consuming.

For this reason I'd love an option to "backup" characters, i.e- export multiple characters as PDFs at once. Even better if we could also do the same for homebrew content to export our custom spells, monsters etc.

This is partly prompted by the recent Wizards of the Coast takeover; while I'm sure nothing is going to change immediately, there's no guarantee it won't eventually in future so it would be nice to be able to backup my content easily.