Add recalculate action to process signature sheets

[MariaCheca]

Context

In some cases when signature sheets are being processed and there's an issue during the process, it stops leaving the rest of the signatures without verification.

What

There should be an action as 'recalculate' to be an option in /admin/signature_sheets/[:signature_sheet_id] in cases as the one described above, so the admins can continue with the processing of the rest of the signatures.

[raul-fuentes]

I'll do this one