Matt McKay

**Note:** The download `notebooks` won't visually work until the s3 location hosts images in `_images`.

~~@AakashGfude for some reason this branch is no longer placing build files in `_build/jupyter` but rather in the root level `_build`. I haven't changed `self.outdir` in the builder so I...

- [x] fix merge conflicts with master branch - [x] the `_downloads` library doesn't seem correct with too many `-1` applied: ``` 3ndp-1.pdf countries-1.csv employ-1.csv iteration_notes-1.pdf realwage-1.csv ticker_data-1.csv aiyagari_obit-1.pdf course_notes-1.pdf...

**Julia:** need to resolve the issue of nested folders now as `uri` are returned from sphinx as: ``` 'getting_started_julia/../_static/figures/julia_term_1.png' ``` - [x] fixed for `images` - [x] fixed for `downloads`...

- [x] test on `lecture-source-py` - [x] test on `lecture-source-jl`

- [x] check `_static` and see if anything is missing (@mmcky) just contains theme support files now.

- [ ] test in live environments. If for example you open `jupyter notebook` on the julia side as `jupyter notebook linear_models` in the `tools_and_techniques` folder then the jupyter notebook...

- [x] check coverage report for `_static`

This PR migrates the static assets to use a flat collection of static objects. 1. all `image` and `figure` directive support files are placed in `_images` 1. all `download` directive...

- [ ] check on `lecture-source-ds` tried to test on the latest `master` today but the compilation stalled -- perhaps on `avalanche.rst` again?