[api] work around mod_xrewrite bug

rack:deflater compresses the head answer (which is in itself smells like a bug)
but mod_xrewrite leaves the content-encoding header in the reply, which leads
to uncompressed answers (from backend) with deflate header
[webui] add default architectures to new repositories

It's adding recommended architectures - of those that are available. which
is i586 and x86_64 for the test suite