Add export to new frontend

[Davidster]

Please merge #70 before reviewing this PR.

This PR is related to issue #45.

Summary

Nothing fancy here.

Updated export function in the backend to support our recent changes (#57) to our sequence JSON spec

• I moved away from our our past decision to parse the JSON request data into POJOs (example here). Simply used our org.json library instead.

  Added a button drop down in the IOPanel which lets you export to .pdf, .md, or .txt.

• It was super easy to support the second two options since our servlet was already creating a .md file prior to converting it to .pdf via pandoc.
• Simply changed the extension of the final downloaded file for the .txt option.

  On the topic of orLists

• If you don't select an option for any of the orLists, no course will be put into the exported file for that entry
• Obviously in the future we will want to warn the user not to export their sequence if it contains issues. Issues could be an unsatisfied pre/corequisite, not enough credits taken, or as I just mentioned an unspecified orList option @PeterGhimself

  Test

  Once again, test on j in case I'm in the middle of testing stuff on d: http://138.197.6.26/courseplannerj/

[PeterGhimself]

For the warning we can use bsModal (bootstrap). I have some experience with that from work at SK 😎👌