Chat commands: block, favorite

[codecat]

Implement /block and /favorite chat commands. For convenience, it should probably accept @ mentions so that they can be autocompleted. For example:

/block @Spammiej
/favorite @Spammiej

Note that on Maniaplanet, @ mentions are normalized to have no formatting at all, to avoid some potential input problems and to simplify the actual chat output. That means that on Maniaplanet we might need to change the blocking/favoriting code to also check for unformatted names.