StealEmojiPolicy: fix String rejected_shortcodes

StealEmojiPolicy: fix String rejected_shortcodes

* rejected_shortcodes is defined as a list of strings in the
  configuration description. As such, database-based configuration was
  led to handle those settings as strings, and not as the actually
  expected type, Regex.
* This caused each message passing through this MRF, if a rejected
  shortcode was set and the emoji did not exist already on the instance,
  to fail federating, as an exception was raised, swiftly caught and
  mostly silenced.
* This commit fixes the issue by introducing new behavior: strings are
  now handled as perfect matches for an emoji shortcode (meaning that if
  the emoji-to-be-pulled's shortcode is in the blacklist, it will be
  rejected), while still supporting Regex types as before.