feat($interpolate): enable escaping interpolated expressions

Previously, Angular would offer no proper mechanism to reveal attempted script injection attacks
when users would add expressions which may be compiled by angular.

This CL enables web servers to escape escaped expressions by replacing interpolation start and end
markers with escpaed values (which by default are `{{{{` and `}}}}`, respectively).

This also allows the application to render the content of the expression without rendering just the
result of the expression.

Closes #5601