Fix dragleave handling to correctly remove active class across browsers

[DVSLabs]

Fixes the dragleave event handler to ensure the PHX_DROP_TARGET_ACTIVE_CLASS is removed correctly across browsers, including Firefox.

See discussion: #4012 (comment)

[SteffenDE]

Can you please add extensive code comments to that area that explain the scenario in detail and why it needs to be this way? E.g. what the different event targets are in that case. The logic is already complex and I'm afraid that with this change it's even more so.

Moreover needing to use closestPhxBinding more than once is quite bad, as it traverses up the whole DOM in the worst case, so I'd like to avoid it if possible.