ability to automatically link to input files

[kecnry]

It would be nice to be able to link to these inline, have them hosted by the website, and also include the link(s) at the top of the tutorial near the download .py and .ipynb links.

• manually or automatically include link(s) in header of ipynb file
• edit copying script to include these files in static/docs/_downloads/<version>/
• website needs to redirect links correctly to hosted version
• update tutorials (at least datasets and passbands) to include links as necessary for all releases
• update online docs for all releases