gravatar: use icon-gravatar for default gravatar sizing and styling instead of icon-empty We may want to use a different style for gravatar then for icon-empty, so we should give it its own class, but still consider it an icon, like the default icon-user is.

# 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
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