Wagtailmenus

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* Adjusted Meta classes on menu item models so that common behaviour is defined
once in AbastractMenuItem.Meta.
* Refactored the AbstractMenuItem's `menu_text` property method to improve code
readability, and better handle instances where neither link_text or link_page
are set.
* Replaced overly long README.md with brand new documentation and a new
README.rst which will render better on PyPi. The documentation is kindly
hosted by readthedocs.org and can be found at
<http://wagtailmenus.readthedocs.io/>
* Added Chinese translations, kindly submitted by 汇民 王 (levinewong)
* Added the 'use_absolute_page_urls' argument to all template tags. When a
value equating to `True` is provided, the menu will be rendered using the
'full URL' for each page (including the protocol/domain derived from the
relevant `wagtailcore.models.Site` object), instead the 'relative URL' used
by default. Developed by Trent Holliday of Morris Technology and Andy Babic.

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* Fixed a bug that would result in {% sub_menu %} being called recursively (
until raising a "maximum recursion depth exceeded" exception) if a
'repeated menu item' was added at anything past the 2nd level. Thanks to
pyMan for raising/investigating.

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* Code example formatting fixes, and better use of headings in README.md.
* Added 'on_delete=models.CASCADE' to all relationship fields on models where
no 'on_delete' behaviour was previously set (Django 2.0 compatibility).
* Marked a missing string for translation (einsfr).
* Updated translations for Lithuanian, Portuguese (Brazil), and Russian.
Many thanks to mamorim, matas.dailyda and einsfr!

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* Added an 'AbstractLinkPage' model to wagtailmenus.models that can be easily
sub-classed and used in projects to create 'link pages' that act in a similar
fashion to menu items when appearing in menus, but can be placed in any part
of the page tree.
* Updated 'modify_submenu_items', 'has_submenu_items' and
'get_repeated_menu_item' methods on MenuPageMixin / MenuPage to accept a
'request' parameter, which is used to pass in the current `HttpRequest`
object the menu is being rendered for.
* Added the `WAGTAILMENUS_SECTION_MENU_CLASS_PATH` setting, which can be used
to override the `Menu` class used when using the `{% section_menu %}` tag.
* Added the `WAGTAILMENUS_CHILDREN_MENU_CLASS_PATH` setting, which can be used
to override the `Menu` class used when using the `{% children_menu %}` tag.
* All `Menu` classes are now 'request aware', meaning `self.request` will
return the current `HttpRequest` object within most methods.
* Added a `get_base_page_queryset()` method to all `Menu` classes, that can be
overridden to change the base page QuerySet used when identifying pages to
be included in a menu when rendering. For example developers could use
`self.request.user` to only ever include pages that the current user has
some permission for.
* Abstracted out most model functionality from `MenuPage` to a `MenuPageMixin`
model, that can be used with existing page type models.
* Added wagtail 1.10 and django 1.11 test environments to tox.
* Renamed `test_frontend.py` to `test_menu_rendering.py`
* In situations where `request.site` hasn't been set by wagtail's
`SiteMiddleware`, the wagtailmenus context processor use the default
site to generate menus with.
* Updated AbstractMenuItem.clean() to only ever return field-specific
validation errors, because Wagtail doesn't render non-field errors for
related models added to the editor interface using `InlinePanel`.
* Added Russian translations (submitted by Alex einsfr).

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* Fixed a bug that would result in {% sub_menu %} being called recursively (
until raising a "maximum recursion depth exceeded" exception) if a
'repeated menu item' was added at anything past the 2nd level. Thanks to
pyMan for raising/investigating.

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* Got project set up in Transifex.
* Updated translatable strings throughout the project to use named variable
substitution, and unmarked a few exception messages.
* Add Lithuanian translations (submitted by Matas Dailyda).
* Better handle situations where `request` isn't available in the context, or
`request.site` hasn't been set.