<content/> tag memory leak.

[bmomberger-bitovi]

In makeInsertionTagCallback the <content> [and possibly can-slots] are set up using addContext, which sets preventDataBindings on the DOM data state of content and slot elements. Since these elements never get added to the DOM, there's no removal mutation triggering a cleanup, and they stay resident in the can-dom-data-state removeDisposalMap.

It should suffice for this case to clean preventDataBindings from the elements after adding context.

[bmomberger-bitovi]

Fixed via some or all of #340, #341, #342, #344