Add ability to exempt urls from transformations

This is a bit flimsy, but when I started looking at applying the
existing transformations to old posts, I found the Paradox forums as an
example of links that became broken after they were processed (because
"fixing" their links ends up breaking them).

This will give a way to exempt any other domains or urls that end up
being a problem, though over the long term it would probably be better
to make this database-based instead of code-based.

tildes/tests/test_url_transform.py

@@ -79,3 +79,10 @@ def test_youtube_unshortened():
     transformed_url = apply_url_transformations(url)
 
     assert transformed_url == "https://www.youtube.com/watch?v=YbJOTdZBX1g&t=1"
+
+
+def test_exempt_url_not_transformed():
+    """Ensure that an exempt url doesn't get transformed."""
+    url = "https://forum.paradoxplaza.com/forum/index.php?forums/518/"
+
+    assert apply_url_transformations(url) == url

tildes/tildes/lib/url_transform.py

@@ -30,6 +30,9 @@ def apply_url_transformations(url: str) -> str:
     """
     parsed_url = urlparse(url)
 
+    if _is_exempt_from_transformations(parsed_url):
+        return url
+
     try:
         parsed_url = _apply_all_transformations(parsed_url)
     except UrlTransformationLoopError:
@@ -39,6 +42,16 @@ def apply_url_transformations(url: str) -> str:
     return urlunparse(parsed_url)
 
 
+def _is_exempt_from_transformations(parsed_url: ParseResult) -> bool:
+    """Return whether this url should be exempt from the transformation process."""
+
+    # Paradox forums use an invalid url scheme that will break if processed
+    if parsed_url.hostname == "forum.paradoxplaza.com":
+        return True
+
+    return False
+
+
 def _apply_all_transformations(parsed_url: ParseResult) -> ParseResult:
     """Apply all relevant UrlTransformer transformations to the url."""
     # Used to keep track of which transformations are restarting the process, so we