[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 generate json directly
instead of recursive to_json

Before project_infos/openSUSE:Factory: 1115ms - after 765ms, so it saves
on that project (with quite some packages) 350ms, which is not bad for
3 lines

Unfortunately JSON.generate can't be used within rails app due to
https://github.com/intridea/multi_json/issues/86 nonsense, so switch
back to yajl
[api] further speed improvements in webui#project_infos:

In Project#expand_all_packages don't bother loading all packages as
ruby objects if all we want to know is package name and project name

reduces runtime from ~590ms to ~370ms (the runtime varies a lot due
to backend parts involved, but I ran before and after each 4 times)

Additionally /source/<prj> also gets a gain

comparing /source/openSUSE:Factory and /source/openSUSE:Factory:Staging
with and without expand:

before
 O:F 727ms  O:F?expand=1 350ms
 O:F:S 9ms  O:F:S?expand=1 8722ms

after
 O:F 87ms OF?expand=1 111ms
 O:F:S 11ms O:F:S?expand=1 79ms

So factor 110 :)

Point of data: /source/openSUSE:Factory from srcsrv takes 172ms here,
/source/openSUSE:Factory:Staging?expand=1 take 240ms