In menu customizer: sortable placeholder not correctly accounting for children height when doing drag and drop

[adamsilverstein]

When dragging and dropping a menu with subitems, the placeholder size is not being correctly calculated. No matter how many subitems exist, the placeholder is always one item high.

I tracked this down to the left hand alignment we use for li items in the customizer layout. removing this just for the placeholder lis resolves the issue.

Here is what dragging with children currently looks like: ​http://cl.ly/412p291a1T1T

Here is the same action with the CSS change (patch incoming): ​http://cl.ly/0A2L0S2q1B26

[adamsilverstein]

Fix a sortable placeholder height issue

[adamsilverstein]

In 32857.diff​:

• Set float: to none for placeholder child li tags, correcting an issue where the placeholder height was always only one item height. After this patch the placeholder size correctly matches the size of the dragged item plus all children.

[ocean90]

In 33028:

Customizer: Fix height calculation of placeholders when dragging menu items.

props adamsilverstein.
fixes #32857.