webadmin: Template main tab columns sorting

This patch adds sorting of fields in Template main tab in webadmin. As a
part of the patch, some existing *Column implementations were adjusted
to extend SortableColumn class.

Change-Id: I8c91564be9bc4119e405282d29a3aa00288126c6
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1104784