fix: don't force rewrite in preview mode

[ascorbic]

Summary

Currently we force a root level rewrite when in preview mode, so that we don't use ODBs or static HTML. This causes a bug where other static files are not found (e.g. images, css etc). This PR removes the force attribute. Because this would mean that statically-generated page files would then be served if using the EXPERIMENTAL_MOVE_STATIC_PAGES option, this also adds forced redirects for these.

Relevant links (GitHub issues, Notion docs, etc.) or a picture of cute animal (small picture)

Fixes #753

[tiffafoo]

[sand] Another test to check that force is true would be good
[sand] Most likely this needs docs

Other than ^ LGTM! Could you include test steps? I've been publishing alpha versions and using them, but I'm guessing there's a better way.

[ascorbic]

@tiffanosaurus Thanks for the review!
Normally we can test most PRs using the plugin's demo site. Otherwise, pointing another site at the repo using package = "../path/to/netlify-plugin-nextjs" should do the trick
Re the docs, I'll try and cover this in the rewrites/redirects doc I'm working on
The test should be covered by the redirects snapshot test, though more specific tests are probably a good plan