ephy-certificate-dialog: reword bad identity error message

This wording should be a bit less confusing.

https://bugzilla.gnome.org/show_bug.cgi?id=733025
ephy-certificate-dialog: remove old code

We do not use the action area now that we have header bars

https://bugzilla.gnome.org/show_bug.cgi?id=733025
ephy-certificate-dialog: fix position of lock icon

If the cert is untrusted then the lock icon displays parallel to the
first row of text; it needs to be centered between them.

https://bugzilla.gnome.org/show_bug.cgi?id=733025
ephy-certificate-dialog: use better default height

Reduce the amount of blank space on this dialog when technical details
are not being displayed. It's just too much. This height is still large
enough that users who want to browse through all the details probably
won't want to resize the window.

https://bugzilla.gnome.org/show_bug.cgi?id=733025