thumbnails: Extract user_uploads_or_external as a function.

This is a prepartory commit for the upcoming changes. It was meaningful
to extract this one out because this function is essentially a condition
check on whether a given url is one of the user_uploads or an external
one. Based on its value we decide whether a url must be thumbnailed or
not and thus this function will also be used in an upcoming commit
patching lib/thumbnail.py to do the same check before thumbnail url
generation.

zerver/lib/thumbnail.py

@@ -18,6 +18,9 @@ from zerver.lib.camo import get_camo_url
 def is_thumbor_enabled() -> bool:
     return settings.THUMBOR_URL != ''
 
+def user_uploads_or_external(url: str) -> bool:
+    return url.startswith('http') or url.lstrip('/').startswith('user_uploads/')
+
 def get_source_type(url: str) -> str:
     if not (url.startswith('/user_uploads/') or url.startswith('/user_avatars/')):
         return THUMBOR_EXTERNAL_TYPE