Molo-commenting

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- Add placeholder attribute on comment textarea widget form

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- Updated notifications templates user-interface for users when comments are replied to by user admin

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- Added notifications for users when comments are replied to
- Added threaded comments that allow all users to reply to comments

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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- ``{% load wagtailmodeladmin_tags %}`` has been replaced by ``{% load modeladmin_tags %}``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