Add Download button for result of a search

[meier-rene]

As requested in issue 350 of MassBank-web it would be a good idea to provide data download for collections in different formats.
For the frontend a download button to retrieve the results in eg. msp format is required.

[sneumann]

Hi, functionality-wise very related, users want to download record sets of the full data, split into positive/negative mode, filtered by MS2 etc. This would possibly be expected on the Content page, not so much on the (spectral) search pages. Yours, Steffen