Improve phpBB3 importer

* Log errors when mapping of posts, messages, etc. fails
* Allow permalink normalizations for old subfolder installation
* Disable importing of polls for now. It's broken.

script/import_scripts/phpbb3/settings.yml

@@ -31,6 +31,9 @@ import:
     categories: true  # redirects   /viewforum.php?f=1            to  /c/category-name
     topics: true      # redirects   /viewtopic.php?f=6&t=43       to  /t/topic-name/81
     posts: false      # redirects   /viewtopic.php?p=2455#p2455   to  /t/topic-name/81/4
+    # Append a prefix to each type of link, e.g. 'forum' to redirect /forum/viewtopic.php?f=6&t=43 to /t/topic-name/81
+    # Leave it empty if your forum wasn't installed in a subfolder.
+    prefix:
 
   avatars:
     uploaded: true  # import uploaded avatars
@@ -50,7 +53,7 @@ import:
   bookmarks: true
   attachments: true
   private_messages: true
-  polls: true
+  polls: false  # Don't set this to true. Importing polls is currently broken.
 
   # When true: each imported user will have the original username from phpBB as its name
   # When false: the name of each imported user will be blank unless the username was changed during import