frontend: memory optimize /packages/ and /builds/ routes
To have properly generated "stream" feeling from streamed jinja, we need
to have streamed sqlalchemy result set as well. So yield_per helped a
lot in case of /builds/.
With packages, it's not that trivial -- we can not use yield_per easily
because we do some in-app processing of the query. But considering that
the number of packages is much smaller than the number of builds, we can
afford it - we only need to limit the number of information given to
the template (so we throw away build.build_chroots basically).
Merges: #1210