Flask-bootstrap

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* Most likely to be the last major Bootstrap 3 release (see
https://github.com/twbs/bootstrap/issues/20631). Make sure you pin
Flask-Bootstrap's version to `<4` to avoid inadvertent updates.

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* IE-specific meta information and viewport meta tags have been added to the
base template.
* The flashed messages are no longer considered "safe", which means HTML-markup
will be escaped. See :func:`flashed_messages` for more information.
* The ``title`` block now contains an empty-by-default title variable.

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* Support for `Flask-Nav <http://pythonhosted.org/flask-nav>`_ added:
:doc:`nav`.
* The sample application has been rewritten to finally resemble a somewhat
educative piece of code.

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* ``action`` finally settled on ``""``.

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* The ``action`` default value of :func:`quick_form` changed to ``.``.
* An ``enctype`` value of :func:`quick_form`, which is the default, will
automatically add the correct type if a :class:`~wtforms.fields.FileField`
is present.
* The main bootstrap css file is no longer linked for just ``media="screen"``.

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* Google Analytics API updated; the legacy API is still available.
* Flashed-message categories are now translated to bootstrap-y categories.