api: Add avatar URL to the GET /profile endpoint.

This endpoint is legacy in a lot of ways, but it seems reasonable that
it should have these data.

templates/zerver/api/get-profile.md

@@ -1,6 +1,6 @@
 # Get profile
 
-Get the profile of the user/bot that requests this endpoint.
+Get basic data about the user/bot that requests this endpoint.
 
 `GET {{ api_url }}/v1/users/me`
 

zerver/openapi/zulip.yaml

@@ -1188,6 +1188,10 @@ paths:
                 allOf:
                 - $ref: '#/components/schemas/JsonSuccess'
                 - properties:
+                    avatar_url:
+                      type: string
+                      description: "URL for the user's avatar"
+                      example: "x"
                     client_id:
                       type: string
                       description: NA
@@ -1226,6 +1230,7 @@ paths:
                       example: 1
                 - example:
                     {
+                        "avatar_url": "https://secure.gravatar.com/avatar/af4f06322c177ef4e1e9b2c424986b54?d=identicon&version=1",
                         "client_id": "74c768b081076fdb3c4326256c17467e",
                         "email": "iago@zulip.com",
                         "full_name": "Iago",

zerver/views/users.py

@@ -518,6 +518,7 @@ def get_profile_backend(request: HttpRequest, user_profile: UserProfile) -> Http
                   client_id      = generate_client_id(),
                   max_message_id = -1,
                   user_id        = user_profile.id,
+                  avatar_url     = avatar_url(user_profile),
                   full_name      = user_profile.full_name,
                   email          = user_profile.email,
                   is_bot         = user_profile.is_bot,