Add REST API functionality for generating menus

[ababic]

Resolves #278.

To do list:

• Implement the API views as GET endpoints
• Implement custom forms and fields for validating GET arguments and supplying useful feedback
• Implement a custom Renderer class to allow view arguments to be updated easily in the Browsable API
• Implement custom Serializer classes to support recursive rendering of menu items
• Implement a way to easily customise serializers for flat/main menu items
• Implement a way to separately customise serialization of sub-menu items in flat/main menus
• Implement a way to easily customise serializers for items in children and section menus
• Add a setting to allow the renderer settings to be controlled
• Add unit tests
• Add documentation

There are endpoints for generating the following menu types:

• main menus
• flat menus
• children menus
• section menus

Each endpoint acts in a similar way to the equivalent template tag, only rendering the menu as JSON instead of to a template. If you're familiar with the tags, you should recognise the options that are available in the API. You just have to provide a few more values, as there's no existing request context to pull existing values from.

The best way to see how they all work is to check out each one in the Browsable API view, where an 'Arguments' form will explain all of the arguments accepted for each view.

Anyone wanting to give this a whirl for themselves should be able to do so by following these instructions:

1. Install the PR version from github using:

   pip install git+https://github.com/ababic/wagtailmenus.git@menus_api
   

2. Add 'wagtailmenus.api' to INSTALLED_APPS in your project's settings
3. If it's not already there, add 'rest_framework' to INSTALLED_APPS in your project's settings
4. Update urls.py for your project to include the wagtailmenus.api.urls, e.g:

   urlpatterns = [
       ...
       url(r'^wagtailmenus_api/', include('wagtailmenus.api.urls')),  # This is new!
       url(r'', include(wagtail_urls)),
   ]
   

5. Load up the v1 index URL in your browser with ?format=api on the end to get the Browsable API view, e.g: http://127.0.0.1:8000/wagtailmenus_api/v1/?format=api

Or, if you want to try things out without interfering with an existing project, you can clone my branch and follow these instructions to get a version up and running:

https://wagtailmenus.readthedocs.io/en/latest/contributing/index.html#developing-locally

[codecov]

Codecov Report

Merging #302 into master will decrease coverage by 4.81%.
The diff coverage is 73.29%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
- Coverage   98.26%   93.44%   -4.82%     
==========================================
  Files          89       94       +5     
  Lines        2945     3465     +520     
==========================================
+ Hits         2894     3238     +344     
- Misses         51      227     +176     

Impacted Files	Coverage Δ
wagtailmenus/settings/base.py	100.00% <ø> (ø)
wagtailmenus/tests/urls.py	100.00% <ø> (ø)
wagtailmenus/utils/misc.py	56.52% <24.00%> (-37.85%)	⬇️
wagtailmenus/api/v1/renderers.py	50.00% <50.00%> (ø)
wagtailmenus/api/v1/serializers.py	56.31% <56.31%> (ø)
wagtailmenus/api/v1/views.py	57.65% <57.65%> (ø)
wagtailmenus/api/v1/forms.py	66.03% <66.03%> (ø)
wagtailmenus/api/v1/form_fields.py	83.92% <83.92%> (ø)
wagtailmenus/api/v1/tests/mixins.py	94.54% <94.54%> (ø)
wagtailmenus/api/conf/defaults.py	100.00% <100.00%> (ø)
... and 49 more

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