Option to delete signature sheets

[decabeza]

Background

Proposals can be supported by signatures, and those supports can be included in the system through the "Signature sheets" menu /admin/signature_sheets In case a sheet is wrongly included (for example associated to the wrong proposal) it can't be deleted from the interface.

What we need

Include a button "Delete sheet and supports" next to each included sheet of supports. When clicking it will delete the supports from the proposal and will mark the sheet as deleted. But will keep showing the information that was shown before deleting, as a record for accountability of the process.