Save XLSForm for form that uses managed encryption

It would be useful to save the uploaded XLSForm when managed encryption is in use, both when encryption is first enabled for a project and when a form draft is uploaded or published after encryption is enabled. This would entail pulling forward the previous XLSForm, similar to what's done in https://github.com/getodk/central-backend/commit/2efdbe989a08b6ab76fbc137e96831380d96c110.

When encryption is first enabled for a project, a new form version is created for each existing form (unless the form uses form-level encryption I think). If the form is published, a new version is created and published. If the form has a draft, the draft is replaced. Right now in both cases, the new version/draft will not have an associated XLSForm even if the previous version/draft did.