Download files with nginx

@antgonza helped with this one.  Remaining to be done:

~~Get nginx into Travis CI~~ (EDIT: We are deferring this for now, since we don't know what to do at this time)
- [x] Add download links in the GUI where appropriate
- [x] Fix whatever tests are about to fail when I submit this PR
Even better -- this will require a change to the `Analysis.all_associated_filepaths` property, but it's minor and is only used in one place right now, so I'll do that.