Collective-messagesviewlet

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- Nothing changed yet.

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- Fix tests & travis build (8)
[laulaz]
- Prevent a bug when compare 2 dates with different timezone format.
[boulch]

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- Fix bug when message configuration don't permit to hide this message (close button stayed).
[boulch]
- Adapted code for Plone5.2/Py3.
[gbastien]

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- Bypass allowed content types contraint when (post)install messagesConfig container
[boulch]

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- Added parameter `caching=True` to `utils.get_messages_to_show`, if `True`,
the method result is cached in the request for given `context`.
[gbastien]

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- Added PseudoMessage class that be be used in viewlet template.
[sgeulette]