Fixes #160 download handling on server

• make download button only appear after PeakFQ analysis is run - outputs not available before then
• make download button disappear when user loads new data, before re-running PeakFQ analysis
• add error handling for if out files don't exist (e.g., nwis peak and nwis_site files only created when input is "Query NWIS")/don't include them in the returned zip
• make download work for Query NWIS txt/psf outfiles
• make PeakFQ analysis work on server for PSF file method
• make download work for upload PSF file txt/psf outfiles
• make download work for single tab-delim file txt/psf outfiles
• figure out plot download handling
• download files to a tmp directory (both within the download handler and within the functions triggered by the run peakfq button)
• fix query NWIS error that occurs on server and makes app crash after running PeakFQ:image.png. I think this is fixed with 6bc3760c, once we push up the latest this should be fixed.