LPS-47692 convert DocumentLibraryUpload to a plugin 

Hey Jon. Mike was my peer reviewer for this one. At first, I sent him all my changes in one commit, but diffs were not readable. I know that we usually like to not make multiple commits for a single file, but this is a good exception. Let me know if there is anything I can do to make the change any more clear. This change is the first step towards the goal of folder drag and drop support. Thank you