Improvements to phpBB3 import script (#10999)

* FEATURE: Import attachments

* FEATURE: Add support for importing multiple forums in one

* FEATURE: Add support for category and tag mapping

* FEATURE: Import groups

* FIX: Add spaces around images

* FEATURE: Custom mapping of user rank to trust levels

* FIX: Do not fail import if it cannot import polls

* FIX: Optimize existing records lookup

Co-authored-by: Gerhard Schlager <mail@gerhard-schlager.at>
Co-authored-by: Jarek Radosz <jradosz@gmail.com>

script/import_scripts/phpbb3/settings.yml

@@ -11,6 +11,73 @@ database:
   batch_size: 1000 # Don't change this unless you know what you're doing. The default (1000) should work just fine.
 
 import:
+  # Set this if you import multiple phpBB forums into a single Discourse forum.
+  #
+  # For example, when importing multiple sites, prefix all imported IDs
+  # with 'first' to avoid conflicts. Subsequent import runs must have a
+  # different 'site_name'.
+  #
+  # site_name: first
+  #
+  site_name:
+
+  # Create new categories
+  #
+  # For example, to create a parent category and a subcategory.
+  #
+  # new_categories:
+  # - forum_id: foo
+  #   name: Foo Category
+  # - forum_id: bar
+  #   name: Bar Category
+  #   parent_id: foo
+  #
+  new_categories:
+
+  # Category mappings
+  #
+  # For example, topics from phpBB category 1 and 2 will be imported
+  # in the new "Foo Category" category, topics from phpBB category 3
+  # will be imported in subcategory "Bar category", topics from phpBB
+  # category 4 will be merged into category 5 and category 6 will be
+  # skipped.
+  #
+  # category_mappings:
+  #   1: foo
+  #   2: foo
+  #   3: bar
+  #   4: 5
+  #   6: SKIP
+  #
+  category_mappings:
+
+  # Tag mappings
+  #
+  # For example, imported topics from phpBB category 1 will be tagged
+  # with 'first-category', etc.
+  #
+  # tag_mappings:
+  #   1:
+  #   - first-category
+  #   2:
+  #   - second-category
+  #   3:
+  #   - third-category
+  #
+  tag_mappings:
+
+  # Rank to trust level mapping
+  #
+  # Map phpBB 3.x rank levels to trust level
+  # Users with rank at least 3000 will have TL3, etc.
+  #
+  # rank_mapping:
+  #   trust_level_1: 200
+  #   trust_level_2: 1000
+  #   trust_level_3: 3000
+  #
+  rank_mapping:
+
   # WARNING: Do not activate this option unless you know what you are doing.
   # It will probably break the BBCode to Markdown conversion and slows down your import.
   use_bbcode_to_md: false