[6.x] Forms 2: Add whereStatus to submission queries

[jasonvarga]

This pull request adds a whereStatus() method to the submission query builder, and uses it to keep partial submissions out of places they were leaking into.

whereStatus() accepts finalized, partial or any. Submissions without a partial key are finalized, so submissions saved before Forms 2 stay finalized.

Like entries, queries return every status unless told otherwise. FormSubmission::query(), Form::submissions() and Form::querySubmissions() include partial submissions, and consumers that only want finalized submissions ask for them explicitly.

Partial submissions were previously included in:

• The form fieldtype's submission counts.
• The {{ form:submissions }} tag. It now outputs finalized submissions by default, the same way the collection tag defaults to published entries. Use status:is="partial" or status:is="any" to override it.

Places that already excluded partial submissions (the submission limit, the forms index count, the webhook and email previews), the submission status filter and the partial submission cleanup job now use whereStatus() instead of querying the partial key directly.

Exporting "All submissions" still includes partial submissions. "Filtered submissions" exports what's in the listing, which is finalized submissions by default.

The SubmissionQueryBuilder contract is unchanged, to avoid breaking existing implementations. Custom submission repositories should add a whereStatus() method to their query builder.