Added madoc middleware

[stephenwf]

This might be better as a standalone package if it could also be used by the config service or other future services

When enabled this will add a "madoc" dictionary to each request object. If a JWT is present it will be parsed (NOT validated)
and the data extracted.

The resulting object will be something like this:

request.madoc = {
  'token': '...', 
  'user': {
    'id': 1, 
    'urn': 'urn:madoc:user:1', 
    'service': False, 
    'service_id': None, 
    'name': 'admin'
  }, 
  'site': {
    'id': 1, 
    'urn': 'urn:madoc:site:1', 
    'name': 'Default site', 
    'gateway': False
  }, 
  'scope': ['site.admin', 'tasks.admin', 'models.admin']
}

There are a few classes that wrap the above structure.

You can use either request.madoc.site.urn or request.madoc.site.id if you want to apply a
pre-applied filter on a site. Note, this will not be usable if request.madoc.gateway is True.

Additionally a recreation of a Madoc internal function that is used for requiring a JWT + scope has been added.

In a view, you can pass in a request object and a list of required scopes.

from madoc.helpers import user_with_scope

class SomeClass:
  def get_some_route(self, request):
    # Will raise a PermissionDenied
    user_with_scope(request, ["site.view"])

    self.some_view_action()

  def get_some_other_route(self, request):
    # Returns the `request.madoc` dict
    madoc = user_with_scope(request, ["site.admin"])

    # Which can be used for sandboxing.
    site_urn = madoc.site.urn

    self.some_admin_action()

To enable, you need to install the middleware in settings.py

MIDDLEWARE = [
    "django.middleware.security.SecurityMiddleware",
    "django.contrib.sessions.middleware.SessionMiddleware",
    "django.middleware.common.CommonMiddleware",
    "django.middleware.csrf.CsrfViewMiddleware",
    "django.contrib.auth.middleware.AuthenticationMiddleware",
+    "madoc.middleware.MadocMiddleware",
    "django.contrib.messages.middleware.MessageMiddleware",
    "django.middleware.clickjacking.XFrameOptionsMiddleware",
]

You can use user_with_scope() if you don't have the middleware installed and it will skip over it. That
may be useful for development or when search is used without JWTs.

Next steps for the library may be to generalise the shape of the above parsed token that could be used
outside of a Madoc context. Also it may be useful to have search specific scopes. (e.g. search.read, search.admin) and
make the "admin" context configurable (If you have site.admin scope, then you have any others).

Fixes #6

Original Typescript implementations

• parse_jwt
• as_user_from_request
• user_with_scope