URI.encode has been removed from Ruby 3

Image caching were not working because the Image model silently decided
all image were "internal": the URI.encode (aka URI.escape) method does
not exist anymore in Ruby 3, which raised an error which where ignored
by the "rescue" line.

Instead of doing "URI.parse(URI.encode(str))", we are just doing
"URI.join(str)", because the join method converts firt the str to
RFC3986.

As this method does not raise error, the "rescue" part has been removed
too.

app/models/image.rb

@@ -46,14 +46,12 @@ def register_in_redis
   end
 
   def internal_link?
-    uri = URI.parse(URI.encode link)
+    uri = URI.join(link)
     !uri.host || uri.host == MY_DOMAIN || uri.host == IMG_DOMAIN
-  rescue
-    true
   end
 
   def blacklisted?
-    uri = URI.parse(URI.encode link)
+    uri = URI.join(link)
     uri.host =~ /^(10|127|169\.254|192\.168)\./
   end