Configure the fact check script with a YAML file.

Instead of configuring the mail fetcher with a Ruby file that we dump
into the `config` directory at deploy time, we can parse a YAML file
with the settings for both the mail fetcher and the logic to generate
and parse fact check addresses. This should give us one single place to
put settings for both sending and receiving these emails.
Set up fact check addresses from a config file.

Instead of having this hard-coded in logic in the `govuk_content_models`
gem (removed in version 11), we set the address pattern up from the same
config file that holds the IMAP settings for the mail fetcher.
Refactor fact check initialiser for clarity.

  * Use `Rails.root.join`, because `Rails.root` is a `Pathname`.
  * Rename the variable holding the parsed YAML, to avoid calling
    several different things `fact_check_config`.
Merge remote-tracking branch 'origin/refactor-fact-check'