views_join should support operator in definition (D8 backport)

ViewsJoin in D8 has an advertised behavior of the JoinPluginBase to permit operators other than '=' in the relationship:

# -- excerpted from core/modules/views/src/Plugin/views/join/JoinPluginBase.php

 * Here are some examples of configuration for the join plugins.
 *
 * For this SQL:
 * @code
 * LEFT JOIN {two} ON one.field_a = two.field_b
 * @endcode
 * Use this configuration:
 * @code
 * $configuration = array(
 *   'table' => 'two',
 *   'field' => 'field_b',
 *   'left_table' => 'one',
 *   'left_field' => 'field_a',
 *   'operator' => '=',
 * );

Currently, the code uses '=' in all cases:

    $condition = "$left_field = $table[alias].$this->field";

To be equivalent to the advertised behavior in D8, it should allow the relationship def to specify an operator:

    $operator = !empty($this->definition['operator']) ? $this->definition['operator'] : '=';
    $condition = "$left_field $operator $table[alias].$this->field";