Support config files specified per branch

[#69707496]
Remove config.sample.yml copying from .travis.yml

[#69707496]

Now we support config files defined per branch.
Add s3-webiste gem, configure it and add deploy scripts

[#69707496]
Add encoded S3 credentials to .travis.yml

[#69707496]
Add deploy task to .travis.yml

[#69707496]
Merge branch 'master' into s3-deploy
Remove mathjax library, use CDN instead

[#69671476]

It's used only on two docs pages, while it consists of 30 000 files,
what is killing S3 deploy.
Merge branch 'master' into s3-deploy