* properly track user modes for context menu
The RPL_ISUPPORT response contains a PREFIX element, which not only tracks the
prefix chars ("@", "+" etc) but also their corresponding mode chars (+O, +v)
This commit changes the context menu to not rely on a hardcoded list but rather
user the one given in the prefix response by the server.
Co-authored-by: Max Leiter <maxwell.leiter@gmail.com>