* 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.
