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.

Additional note: it would be interesting to properly catch MRF exceptions and log them to more easily catch those kinds of issues, as they could easily cause complex federation issues and headaches. This one took weeks to figure out!