Move images out of /public and into the asset pipeline

An oddity here is that I had to rename logo.png to
alaveteli-logo.png, or the version from the alaveteli
repository was used after precompiling the assets.
(This suggests that generation of precompiled assets
with "rake assets:precompile" is picking the wrong
version; i.e. not going through the asset path in
reverse order.)