Ensure medium ID's uniqueness

found a case where different medium-editor instances could be assigned the same ID if the first instance was deactivated, then activated (it would get re-assigned to an already-assigned ID).

this changeset also fixes a few cases where an ID was being assigned to a smaller piece of medium-editor (e.g. `#medium-editor-toolbar-actions` without a unique identifier being appended to the end of that smaller piece)