Unify talks URLs in emails, for private messages

[benoitmasson]

When a reviewer sends a private message about a talk proposal, an email is sent to all admins and reviewers.

Depending on who wrote the message, the URL contained in the email differs:

In the screenshot above, email from Loïc leads to …/admin/sessions/… while email from Benoît leads to …/sessions/….

The admin link should be prefered (for private messages only), since it leads directly to the "review" tabs.