The Security Information dialog (details about GPG signature etc.) doesn't
have scroll bars when it contains too much information to fit.

Steps to reproduce the problem:
1. Open a mail message signed with a GPG key.
2. Click on the Security Information button under that message.
3. If there's a lot of information (such as when importing a new public
key), it doesn't fit in the dialog.

Actual Results:
The information may not fit in the dialog, and the dialog doesn't provide
scroll bars to mitigate the problem.

Expected Results:
Scroll bars when the information doesn't fit in a reasonably sized Security
Information dialog.