Split `select_files` route

[rhsimplex]

https://github.com/merantix/picasso/blob/master/picasso/picasso.py#L298

Select files is actually file selection AND result display. This should be split up not only for conceptual simplicity, but also because we'll want to do cleanup after the rendering (since both GET and POST work at this route but represent different functionality, we can't simplay apply an @after_request decorator e.g. http://flask.pocoo.org/snippets/53/).