better build status json

currently the succededBuilds and failedBuilds fields could use some improvements:

failedBuilds: contians lists of the format [appid, versionCode, errorlog]. Let's make this explicit by making it a dict: {'appid': appid, 'versionCode': versionCode, 'log': errorlog}

successfulBuilds: is just dumping a copy of the current app object to json which is telling us exactly nothing about which builds exactly did succeed. Let's add a similar structure to the above instaead: {'appid': appid, 'versionCode': versionCode}

Additionally I'd like to add links to full build logs to both when these are enabled.

This could look like this: {'appid': appid, 'versionCode': versionCode, 'logfile': "https://f-droid.org/repo/<appid_versionCode.log.gz>"} or alternatively we don't include the full link but just the part relative to /repo.