Allow purge the harvest source

[LittleRed945]

Make a new list item that can purge the harvest source so that one can directly delete the data in the harvest source and package table when a harvest source is no longer needed.

[LittleRed945]

Hi @amercader , could you give some comments to this PR? Thanks!

[LittleRed945]

Run paster --plugin=ckan db init -c test.ini
Traceback (most recent call last):
  File "/usr/bin/paster", line [11](https://github.com/ckan/ckanext-harvest/actions/runs/3226605184/jobs/5334913445#step:6:12), in <module>
    load_entry_point('PasteScript==2.0.2', 'console_scripts', 'paster')()
  File "/usr/lib/python2.7/site-packages/paste/script/command.py", line 102, in run
    invoke(command, command_name, options, args[1:])
  File "/usr/lib/python2.7/site-packages/paste/script/command.py", line [14](https://github.com/ckan/ckanext-harvest/actions/runs/3226605184/jobs/5334913445#step:6:15)1, in invoke
    exit_code = runner.run(args)
  File "/usr/lib/python2.7/site-packages/paste/script/command.py", line 236, in run
    result = self.command()
  File "/srv/app/src/ckan/ckan/lib/cli.py", line 357, in command
    self._load_config(cmd!='upgrade')
  File "/srv/app/src/ckan/ckan/lib/cli.py", line 330, in _load_config
    self.site_user = load_config(self.options.config, load_site_user)
  File "/srv/app/src/ckan/ckan/lib/cli.py", line 237, in load_config
    load_environment(conf.global_conf, conf.local_conf)
  File "/srv/app/src/ckan/ckan/config/environment.py", line 116, in load_environment
    p.load_all()
  File "/srv/app/src/ckan/ckan/plugins/core.py", line 140, in load_all
    load(*plugins)
  File "/srv/app/src/ckan/ckan/plugins/core.py", line [15](https://github.com/ckan/ckanext-harvest/actions/runs/3226605184/jobs/5334913445#step:6:16)4, in load
    service = _get_service(plugin)
  File "/srv/app/src/ckan/ckan/plugins/core.py", line 256, in _get_service
    return plugin.load()(name=plugin_name)
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2443, in load
    return self.resolve()
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2449, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/__w/ckanext-harvest/ckanext-harvest/ckanext/harvest/harvesters/__init__.py", line 1, in <module>
    from ckanext.harvest.harvesters.ckanharvester import CKANHarvester
  File "/__w/ckanext-harvest/ckanext-harvest/ckanext/harvest/harvesters/ckanharvester.py", line 7, in <module>
    from urllib3.contrib import pyopenssl
  File "/usr/lib/python2.7/site-packages/urllib3/contrib/pyopenssl.py", line [23](https://github.com/ckan/ckanext-harvest/actions/runs/3226605184/jobs/5334913445#step:6:24), in <module>
    from ndg.httpsclient.ssl_peer_verification import SUBJ_ALT_NAME_SUPPORT
ImportError: No module named ndg.httpsclient.ssl_peer_verification
Error: Process completed with exit code 1.

Does anyone have any suggestions to solve the ckan2.8 failure problem?