feat: set course for wiki based on the wiki_slug

Learners want to have the usual course navigation when viewing a wiki,
so that they can go back to the course related to the wiki and browse
other tabs/sections of the course.

Wiki reads the course from the `request.course`. If it's not present,
i.e.  None or not set on the request, it will not show the course
navigation UI.

It seems like `WikiAccessMiddleware` already has the code that parses
course id from the request (when the request is for a wiki view) and
sets the course for the request. However, it doesn't work in most
scenarios, because the course id is not in the it's normal format in
most requests that go to wiki.

For example, when a leaner clicks on a wiki tab from the course
overview, they are redirected to `/wiki/<wiki_slug>/` path. The wiki
slug is taken from course's `wiki_slug` field. This slug can be used to
figure out what course this wiki belongs to in most (not all) cases.

This commit adds code to the `WikiAccessMiddleware` that attempts to
find a course based on wiki slug, and in case of success, sets the
course to the `request.course`, so that wiki can display course
navigation UI.

lms/djangoapps/course_wiki/tests/test_middleware.py

@@ -34,3 +34,10 @@ class TestWikiAccessMiddleware(ModuleStoreTestCase):
         response = self.client.get('/courses/course-v1:edx+math101+2014/wiki/math101/')
         self.assertContains(response, '/courses/course-v1:edx+math101+2014/wiki/math101/_edit/')
         self.assertContains(response, '/courses/course-v1:edx+math101+2014/wiki/math101/_settings/')
+
+    def test_finds_course_by_wiki_slug(self):
+        """Test that finds course by wiki slug, if course id is not present in the url."""
+        response = self.client.get('/wiki/math101/')
+        request = response.wsgi_request
+        self.assertTrue(hasattr(request, 'course'))
+        self.assertEqual(request.course.id, self.course_math101.id)