Enforce no formatting changes for PRs via CI (GH-2951)

Now PRs will run two diff-shades jobs, "preview-changes" which formats
all projects with preview=True, and "assert-no-changes" which formats
all projects with preview=False. The latter also fails if any changes
were made.

Pushes to main will only run "preview-changes"

Also the workflow_dispatch feature was dropped since it was
complicating everything for little gain.