Warning for line and area self-intersection

[quincylvania]

iD tries to prevent users from creating self-intersecting ways, but in some cases it is still possible (#4745). Users may also come across existing self-intersecting ways from various sources. iD should flag any self-intersections as errors and provide quick fixes.

This is separate from layer-based validation issues such as highway-highway crossings without bridges or intersections.

See #5830 for upcoming UI improvements that will surface issues immediately as you edit.

Re: #4787