accept AppImages from bitbucket downloads, second attempt

Since you closed #3211, I open a new issue: you applied the patch which removed my contribution and closed the issue, but this does not resolve it. Without the bitbucket-specific code, bitbucket repos with download section will not result in a download button. I suggested you test the remainder of the code after applying the patch, to make sure that removal of my code does indeed remove the problems you encountered with non-bitbucket catalog entries, but you did not report back about that, and instead just left the tentative patch in place, effectively disabling my code. I realize that bitbucket repos occur only in a small percentage of the catalog entries, so maybe you prefer not having specific code for them after all? I could live with that - after all the download section is just a click away from the main page of the repo. What do you say? If you do want the bitbucket code after all, I can send a patch to un-comment it, if you don't want it, we might as well remove the commented-out code altogether.