Long‐term follow‐up of a randomized, double‐blind, phase <scp>III</scp>, multi‐centre study to evaluate the safety and efficacy of field‐directed photodynamic therapy (<scp>PDT</scp>) of mild to moderate actinic keratosis using <scp>BF</scp>‐200 <scp>ALA</scp> versus placebo and the <scp>BF</scp>‐<scp>RhodoLED</scp>® lamp