[linear-classify-demo]: Fix dead link and embed demo locally into repo.

[andrei-simion]

The linear classification notes linked to an interactive demo at:
http://vision.stanford.edu/teaching/cs231n/linear-classify-demo
which is now a dead URL. The demo seems to be exist here:
http://vision.stanford.edu/teaching/cs231n-demos/linear-classify/

This commit embeds the demo directly in the repository under:
assets/linear-classify-demo/
mirroring the existing pattern used by:
assets/conv-demo/

Demo source attribution / original URL:
http://vision.stanford.edu/teaching/cs231n-demos/linear-classify/
The now dead link was last updated in commit ed95fdf

This avoids issues with future broken external URLs as the demo is self-contained within the repo. And by version controlling the demo, future contributions can be made to the demo itself.

The link in linear-classify.md is updated to a relative path so it remains valid regardless of hosting.

Changes to the original demo source have been kept minimal:

• MathJax CDN: http://cdn.mathjax.org/mathjax/latest/MathJax.js changed to https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js (cdn.mathjax.org was shut down and no longer resolves)
• Google Fonts: http:// upgraded to https://

All other source code is unchanged from the original. e.g.

$ wget -qO- "http://vision.stanford.edu/teaching/cs231n-demos/linear-classify/" | diff - assets/linear-classify-demo/index.html
7c7
<   src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
---
>   src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
17c17
< <link href='http://fonts.googleapis.com/css?family=Open+Sans:400,300' rel='stylesheet' type='text/css'>
---
> <link href='https://fonts.googleapis.com/css?family=Open+Sans:400,300' rel='stylesheet' type='text/css'>