celery: move send_email task to a better home in notification model Avoid bundling everything from many different layers in one big task library. This is more feasible now when we don't need kallithea.CELERY_APP set at import time.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.