Drop jekyll remote theme and simplify configuration

[onerandomusername]

due to our usage of safe: true
the plugins field was NEVER used in the configuration file
see https://jekyllrb.com/docs/plugins/installation/

this simplifies our dependencies and uses the gem
for our theme rather than using a remote theme, which means
one less dependency to rely upon

[artlogic]

Unfortunately remote theme is required for GitHub pages unless the theme is supported, and ours isn't.

[onerandomusername]

We can simply drop the remote theme extension if we proceed with implementing #230, which also means that our local environment can be nearly equivalent to our build.