Encode slashes in challenge_name, closes #81

A slash in the `challenge_name` of a MapRoulette challenge is valid and therefore has to be encoded. `requests` can't do this for us because we pass it the full URL (which contains slashes that must not be encoded).

Author: rYR79435

maproulette/api/challenge.py

@@ -51,7 +51,7 @@ def get_challenge_by_name(self, project_id, challenge_name):
         :param challenge_name: the name corresponding to the challenge
         :returns: the API response from the GET request
         """
-        response = self.get(endpoint=f"/project/{project_id}/challenge/{challenge_name}")
+        response = self.get(endpoint=f"/project/{project_id}/challenge/{challenge_name.replace('/', '%2F')}")
         return response
 
     def get_challenges_by_tags(self, challenge_tags, limit=10, page=0):