Collective-confirmableforms

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- Remove implementedOrProvidedBy import from PloneFormGen that was only meant
for Z2 interfaces compatibility. Removed from PFG 1.7.7.
[fredvd]

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- Make the plain_mail and html_mail fields optional, but give a
validation error when neither of them is filled in.
[maurits]

- Add a field to select a different thank-you page to show when the
form has been confirmed. Fall back to the standard thank-you page
of the form or the fg_result_view page.
[maurits]

- When there is no plain text or no html text, do not send that part.
[maurits]

- Do not use a RichTextWidget for the html field. Any tal tags
would get stripped.
[maurits]

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- Translate the Confirmation schemata tab.
[maurits]

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- Time during which the form can be confirmed is now a setting in the
confirmer. [vincent]

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- Fixes with schemata that was causing error when saving our custom CT
objects. [vincent]

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- Sender email is not mandatory anymore. We use the one set in Plone
if no address is given. [vincent]

- Dutch translations. [jladage]

- Fixes in labels and display in the edit form. [vincent]