Molo-polls

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- Pin molo core to >=5.0.0

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- Added multi-site functionality for polls

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- Added Polls load_polls template tag and new polls_details template file for polls footer headline link

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- Removed ability to delete Polls IndexPage in the Admin UI

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- Updated templates in order to reflect styling changes in modeladmin

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- Removed dependency on wagtailmodeladmin

Backwards incompatible changes
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- Removed use of ``wagtailmodeladmin``: use ``wagtail.contrib.modeladmin`` instead
- ``{% load wagtailmodeladmin_tags %}`` has been replaced by ``{% load modeladmin_tags %}``

NOTE: This release is not compatible with molo versions that are less than 4.0