imo light has an unmeasurable amount of mass.
No, it does not. Otherwise all the light would bend towards the ground.
Large amounts of gravity can distort light, but by very slim amounts. That has to do with more complicated stuff, though.
photons move in straight paths always unless of course acted on by gravity. earth has gravity and last time i checked mass has gravity. earth has very very large mass, therefore lots of gravity so in turn, earth does affect light.
Black holes for example are massive gravity wells, they have so much mass and are so dense, most light cannot escape it.
Mass isn't what's pushing the sails though, it's energy. Moving energy. That's what light is, energy.
everything is energy. mass can be converted to energy (heat energy, friction, etc). A good example of this too is fission. When we break down uranium-transuranium elements, they split into two different elements, but if you were to add the mass of both elements up it would not equal the mass of the previously fissioned element. This proves that mass was transformed into energy, or heat.