FIX: improvements for vanilla bulk import (PR #10212)

Adjustments to the base:

  1. PG connection doesn’t require host - it was broken on import droplet
  2. Drop topic_reply_count - it was removed here -
  3. Error with backtrace.join("\n") -> e.backtrace.join("\n")
  4. Correctly link the user and avatar to quote block

Adjustments to vanilla:

  1. Top-level Vanilla categories are valid categories
  2. Posts have format column which should be used to decide if the format is HTML or Markdown
  3. Remove no UTF8 characters
  4. Remove not supported HTML elements like font span sub u