allow new_study_prefix config var

Uses a config variable (`new_study_prefix` in the `[apis]` section of `private/config`) to tell the Phylesystem what prefix to use when creating a new study.
requires the branch that is the subject of https://github.com/OpenTreeOfLife/peyotl/pull/31

running `ws-tests/test_study_create_delete.py` will print out the ID. It should match what you put in the config file (note that it should be an `[a-z][a-z]_` prefix).