Allow drag item inside drop list to be pinned

#### Please describe the feature you would like to request.
It would be nice if we could pin some drag items inside a list, this way sorting wouldn't have any effect on these elements.

#### What is the use-case or motivation for this proposal?
If sorting is activated on a list, it does affect all drag items, including disabled ones. We can't prevent a disabled drag item to be sorted too.

#### Is there anything else we should know?

No files or tests are named in the issue. Start by locating Angular's drag-and-drop list sorting behavior and determine how disabled drag items are currently handled; done should allow designated items to remain pinned while other items are sorted.

Written by the indexing model from the issue text.