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
reduces this some if:

```python
constructor_and_ids = [(RawData, raw_data_id), (PreprocessedData, preprocessed_data_id), (ProcessedData, processed_data_id)]
for constructor, id_ in constructor_and_ids:
    filepath_ids.extend(_get_study_fpids(constructor, id_)
```