[WIP] Implement alphabetical subject listing as `@@subject-listing`.

:construction: 

Adds a subjects listing view like: http://www.bern.ch/bern_index
The Plone default `Subject` is used.

Before merging I need to boot Plone and see how it looks :wink: 

@senny could you take a look at the testing? I'd appreciate some feedback :wink: 

/cc @phabegger