Introduce some fuzz to frequently reappearing Chromatik requests

[davidfoerster]

We have some frequently reoccurring images due to equally frequently reoccurring search requests to Chromatik. From experience we know, that these requests usually result from emotionally completely neutral phrases with grey colours and no keywords.

To prevent the most frequently reoccurring images, we should introduce some fuzz into either the search requests or the selection of result subsets. E. g., we could randomize the value of the start parameter of the search to something different than 0 in these cases.

[davidfoerster]

Implemented in fc1b820.