Collective-contact-plonegroup

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- As vocabulary `ram.cache` cachekey are generated based on
module/class names, renamed vocabularies
`collective.contact.plonegroup.selected_organization_services` to
`collective.contact.plonegroup.browser.settings.SelectedOrganizationsElephantVocabulary`
and `collective.contact.plonegroup.sorted_selected_organization_services` to
`collective.contact.plonegroup.browser.settings.SortedSelectedOrganizationsElephantVocabulary`.
Added tests showing that cache is correctly invalidated when selected organizations changed.
[gbastien]

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- Use `collective.contact.plonegroup.sorted_selected_organization_services`
for `settings.IFunctionSchema.fct_orgs`
(field for restricting a suffix to some organizations).
[gbastien]
- In `validateSettings` invariant check if Plone group is empty using
`getGroupMembers` that ignores `<not found>` users instead `getMemberIds`.
[gbastien]

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- Added group_as_str param in organizations_with_suffixes function.
[sgeulette]

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- Added parameter `caching=True` to `utils.get_organization`, this will store
the organization object in the REQUEST and avoid too much catalog queries.
[gbastien]

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- When passing `kept_org_uids` to `utils.get_organizations`,
make sure order is preserved.
[gbastien]

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- Added `BaseOrganizationServicesVocabulary._term_title` to ease override of
rendered term title.
[gbastien]