[17.0][IMP] purchase_request (Lines readonly if is_editable==False)

[patrickrwilson]

Majority of the fields are readonly once the PR gets approved except the product lines which can be edited even though the PR was approved or done. This PR adds the same readonly="is_editable == False" that the other fields use on the form to the line_ids to not allow modifications once the PR is approved/done.

[github-actions[bot]]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days. If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.