ssh: introduce ini setting 'ssh_enabled', disabled by default Administrators should control the use of SSH and may want to disable SSH access, temporarily or permanently. An explicit setting ssh_enabled is better than e.g. checking for a valid ssh_authorized_keys setting, to allow such trivial temporary disabling. To keep the controllers simple, introduce a decorator IfSshEnabled instead of repeating the same config checks in every method.

# 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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