Feat: Add web form to request dark

@ahmedk

Currently dark processing is performed for all detectors of a given instrument. The check is performed only for precence of data. However in some cases data could be present but not suitable for evaluation of dark constants.

Therefore a new parameter: detectors is introduces in a script request_darks. This may make a script complicated and inconvenient to use. This MR introduces a web form to request dark.