settings: Rename "user avatar" to "profile picture"

This renames references to user avatars, bot avatars, or organization
icons to profile pictures. The string in the UI are updated,
in addition to the help files, comments, and documentation. Actual
variable/function names, changelog entries, routes, and s3 buckets are
left as-is in order to avoid introducing bugs.

Fixes #11824.

zerver/tests/test_upload.py

@@ -1055,7 +1055,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
 
     def test_delete_avatar(self) -> None:
         """
-        A DELETE request to /json/users/me/avatar should delete the user avatar and return gravatar URL
+        A DELETE request to /json/users/me/avatar should delete the profile picture and return gravatar URL
         """
         self.login(self.example_email("hamlet"))
         hamlet = self.example_user('hamlet')