Add some numerics to the 'Just display' list.

These are all defined in RFC 2812, but were not displayed as server
messages by quassel:
256 (RPL_ADMINME), 257 (RPL_ADMINLOC1), 258 (RPL_ADMINLOC2),
259 (RPL_ADMINEMAIL) and 263 (RPL_TRYAGAIN, this one is technically an
error, thus shown as such).