framework/sealert: add "Last Seen" column to alert list
Add a new column showing the date & time when the denial was last seen
for convenience.
Note that the "Last Seen" string is already translated so this doesn't
require a translation update.
Resolves: https://pagure.io/setroubleshoot/issue/52
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>