[api] need to check given md5sum against srcmd5 and verifymd5

to be on the safe side, we check for both
[webui] remove file no longer needed
[api] fix the latest migration to the actual code

I screwed that up and test suite don't test migrations ;(
[api] fix performance problem when rendering json

as it turned out rails is forcing a specific (slow) json render, so
it doesn't matter how fast the json gem is if it's not used. So
overwriting the render used for render: json to use fast_generate