templates.flights.list_json: add alt-text for pinning tooltip
templates.flights.list_json: add alt-text for pinning tooltip

Needed due to revert of f8381acde4c52e83b9908aa83c4e6d21ecd39c6f

Harry-R/skylines,snip/skylines,TobiasLohner/SkyLines,skylines-project/skylines,RBE-Avionik/skylines,snip/skylines,Harry-R/skylines,kerel-fs/skylines,snip/skylines,RBE-Avionik/skylines,kerel-fs/skylines,dkm/skylines,dkm/skylines,Turbo87/skylines,kerel-fs/skylines,RBE-Avionik/skylines,shadowoneau/skylines,RBE-Avionik/skylines,shadowoneau/skylines,dkm/skylines,Turbo87/skylines,shadowoneau/skylines,Turbo87/skylines,skylines-project/skylines,Harry-R/skylines,skylines-project/skylines,dkm/skylines,skylines-project/skylines,Turbo87/skylines,TobiasLohner/SkyLines,TobiasLohner/SkyLines,Harry-R/skylines,shadowoneau/skylines