blueslip: Protect against drafts pitfall for rows.id.

If folks use an overly broad selector for message rows,
they will accidentally include drafts from the drafts
dialog, which won't have zids.  More specific selectors
will be more efficient and possibly prevent strange
behaviors.

For testing convenience, we extract the message.

frontend_tests/node_tests/lightbox.js

@@ -14,6 +14,8 @@ set_global('popovers', {
     hide_all: () => {},
 });
 
+rows.is_draft_row = () => false;
+
 set_global('$', global.make_zjquery());
 
 run_test('pan_and_zoom', () => {

static/js/rows.js

@@ -41,7 +41,16 @@ exports.last_visible = function () {
     return $('.focused_table .selectable_row').last();
 };
 
+exports.is_draft_row = function (row) {
+    return row.find('.restore-draft').length >= 1;
+};
+
 exports.id = function (message_row) {
+    if (exports.is_draft_row(message_row)) {
+        blueslip.error('Drafts have no zid');
+        return;
+    }
+
     /*
         For blueslip errors, don't return early, since
         we may have some code now that actually relies
@@ -49,6 +58,7 @@ exports.id = function (message_row) {
         that up in the future, but we mainly just want
         more data now.
     */
+
     if (message_row.length !== 1) {
         blueslip.error("Caller should pass in a single row.");
     }