ENH: Add taxa summaries view

I've added a `TaxaSummaryHandler` class that takes care of displaying the
taxa summaries results (if available) and provides a link for the user to
download the text file.

Note that I've changed `webserver.py` to point to the correct data path using
`AMGUT_CONFIG.base_data_dir` so that users can access files going to
`localhost:7777/results/taxa-summaries/00000001.txt`.

Make sure your configuration file is pointing to a directory with a taxonomy
summary.

For now I've set it to be
`AMGUT_CONFIG.base_data_dir/taxonomy-summaries/00000001.txt` and so on.