[backend] checkpatchinfo: adapt for channel sources

Channels do not have logfiles.
[backend] checkchannel: deal with packages in maintenance projects having a name suffix
[backend] break channel if a binary for a package is missing
[webui] do not call into API for every test case

There is no more need to clear the internal API cache
[webui] remove issue models
[webui] switch the default back to transactional fixtures

It's much faster and most test cases don't need the cleanup any longer
[webui] remove some more XML models