Allow admins to edit all talks

[maser]

The app currently only allows you to edit your own talks. Admins need to be able to edit talk in order to anonymize them for the selection process.

Changes:

• Allow admins to edit talks at any time, even after the call closes.
• Add a link to the edit-talk page from the admin proposal page.

Hints:

• admins: staff attribute on User is true.
• TalksController has all the access control.
• Admin::ProposalsController#show page should have the link.