Separate CE/EE code in board_list_header.vue

app/assets/javascripts/boards/components/board_list_header.vue implements EE features and the component needs to be refactored to exclude the EE feature implementations.

example:

// app/assets/javascripts/boards/components/board_list_header.vue
        <!-- EE start -->
        <span
          v-if="listType !== 'label'"
          v-gl-tooltip.hover
          :class="{
            'gl-display-block': list.collapsed || listType === 'milestone',
          }"
          :title="listTitle"
          class="board-title-main-text gl-text-truncate"
        >
          {{ listTitle }}
        </span>
        <span
          v-if="listType === 'assignee'"
          v-show="!list.collapsed"
          class="gl-ml-2 gl-font-weight-normal gl-text-gray-500"
        >
          @{{ listAssignee }}
        </span>
        <!-- EE end -->