Enable deployment of demo site to Github pages

[broadwell]

Target URL would be https://sul-cidr.github.io/scriptchart

[broadwell]

This one is close to being done, at least as a first instance. The primary changes involved were:

1. adding the github-pages Jekyll plugin Ruby gem to the Gemfile; this basically locks all other gems to versions that are supported by Github pages (so some other gems had to be downversioned as a result), and
2. adding the gh-pages Node module to package.json and a further entry in 'scripts' to enable the use of gh-pages to deploy a built site (built via 'yarn build') to the github-pages branch associated with the scriptchart repo.

[broadwell]

Next steps: consider creating multiple deployment targets on Github Pages, one for a stable/staging version of the site (perhaps use the existing scriptchart path for this) and one that is used to demonstrate and test experimental features (perhaps scriptchart-dev).