Move to Giscus for comments and set up docker development environment

[alanplotko]

• Removes custom yaml jobs to leverage Jekyll's merging of yaml configs
• Move from Staticman to Giscus as a trial run, retain app integration in case of future revert
• Set up development environment using Docker compose
• Use unlighthouse for site-wide lighthouse testing
• Leverage latest gulp version and redesign build jobs
• Use browsersync when developing locally