Petition Signature: add an Edit link for admins

[mlutfy]

Overview

Contribution Pages and Event Register pages have a handy "Configure" button at the top, so that admins can quickly edit that page.

This PR adds the same feature for the Petition Signature form.

Before

Confused admins: "how do I edit this page"?

After

[civibot]

(Standard links)

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[colemanw]

IMO this is an easy merge as the button will only be available to admins.

[yashodha]

@mlutfy @colemanw merging this. Thanks!