Migrates email user options to a new data structure, where `email_always`, `email_direct` and `email_private_messages` are replace by

* `email_messages_level`, with options: `always`, `only_when_away` and `never` (defaults to `always`)
* `email_level`, with options: `always`, `only_when_away` and `never` (defaults to `only_when_away`)

* 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.

- Extract name from profile_fields_data table
- Use 'posts.poster_ip' to track topic views

This fixes the conversion for quotes, code blocks and lists (except for nested lists). It also discourages the usage of the ruby-bbcode-to-md gem.

* decode html entities within code blocks

* Only import users that actually participated in the bbpress part of Wordpress; import password hashes

* create permalinks for topics

* Better handling of [code] blocks

Thanks to Leo Davidson for the improved regex.
https://meta.discourse.org/t/importing-from-phpbb3/30810/153

Related to https://meta.discourse.org/t/undefined-local-variable-or-method-e-during-phpbb3-migration/48861

* phpBB3 importer: ignore query parameters in internal links

* phpBB3 importer: Don't fail if guest usernames differ only by case

* phpBB3 importer: Import username as name for guest users

* Reconnect to phpBB3 database on connection loss

* Map geek smiley to :nerd: emoji in phpBB3 importer

* Import PMs to yourself from phpBB3

* Allow empty table prefix in phpBB3 importer

In addition this tries to automatically fix PMs that were migrated
from phpBB2 to phpBB3.

Fix import of polls from phpBB3

http://use-the-index-luke.com/no-offset

There are a few changes to the original permalink import:
- adds permalinks for posts
- does not create incorrect permalinks for private messages
- adds permalink normalizations for topics and posts

phpbb-import-script: move bbcode_to_md to before other text processing

This includes all new emoji from Unicode 7 and Unicode 8 with the
exception of skin tones which will require a new interface.

This seems to fix the issue I reported at
https://meta.discourse.org/t/import-script-phpbb/40424

- Ruby doesn't support the $2y$ prefix used for bcrypt Blowfish hashes.
- The importer ignored users that didn't have optional profile fields.

FIX: phpBB3 importer should ignore unknown topic_id in internal link

FIX: phpBB3 importer failed to import users

FEATURE: Skip batches if posts or messages exists

Update all import scripts to take advantage of all_records_exist?