Allow completely removing the CMS on build

[jimafisk]

It would be great to implement a --cms flag for the CLI tool that we could set to false to completely remove all CMS files from build.

This would be useful not only for folks who want a simple SSG without a CMS, but also for people who want to make edits only on a protected URL and do prod deployments without editing capabilities. For example, you could have a site like edit.mysite.com that sits behind GitLab auth that users make content changes to, then merging into the main branch could kick off a plenti build --cms=false build that deploys to mysite.com that is publicly available. This would increase both the security and performance of the prod site.

We would have to change the static import statements in router.svelte to dynamic imports and account for the default user and adminMenu props that end users could potentially be using in their apps.