Provide an explicit path for the deployment of individual themes

Rather than mixing terminology with public and private, and relying
on groups of public and private plugins, we can separate some of the
requirements by allowing the deployment of individual themes in a
very specific way.
Provide an explicit path for the deployment of individual plugins

For the same reasons as the previous commit, we want to explicitly
account for the deployment of individual plugins while maintaining
other deploy routes for groups of public and private plugins.
Correct old instances of theme-public to theme-individual
Account for the deployment of private plugin groups
Account for groups of private themes
Add drop down options for new private plugin and theme groups