Migrate to SCSS

[Leland]

Ultimately, this PR is about cleaning up some very old assets, moving to a modular SCSS approach, and thus clearing the way for far easier styling without needing to deeply override Bootstrap.

This also comes with other benefits, like not shipping unused Bootstrap features, and sourcemaps:

Major

• Removes jQuery UI entirely, as it appears unused
• Converts custom styles into SCSS, and moves Bootstrap and the Bootswatch theme to the same, fixing visual inconsistencies as a result
• Upgrades from Bootstrap v3.3.1 to v3.4.1, the final 3.x release
• Moves assets (CSS, JS, audio, images) to the directory structure noted on the Jekyll docs:

  .
    ├── assets
    │   ├── css
    │   ├── images
    │   └── js
    │   └── audio
    ...
    

Minor

• Introduces a .editorconfig file
• Reformats all JS, CSS, and HTML that is not from a vendor using the default Prettier config
• Moves vendored JS into a dedicated folder
• Moves CDN-based files into vendorized assets
• Remove LICENSE and README.md from production site
• Remove unused Bootstrap partials from full output

WIP, more coming.

[Leland]

The scope of this grew a bit.

I'll split out the Bootstrap SCSS work to its own PR.