Line item checkbox IDs are numeric; need prefix

Filing this separately from bug 2009092, because this won't break anything essential, but we do need to fix it: the line item checkbox IDs in the PO list are numeric. For various reasons, HTML ID attributes need to start with a letter. (See https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/id for the details.)

[id]="li.id()" should be something more like [id]="'lineitem-'+li.id()"

... however, I'm not confident I can change this here without fouling up the logic elsewhere, so I'll let someone else tackle this.

Standardizing our DOM ID attributes would help us avoid these errors; see bug 2018410.