validate any associations on an extension

The validator only checked if the extension was associated to a line
where as we don't want to delete it if it's associated to anything
(queue, group, conference, etc)
rename validator
make sure extension isn't already associated

we don't want to associate an extension with more than one line
add extension_dao.get_type_typeval

needed by an extension validator
Merge branch '5105_extension_association'