IRCClient: Add nick_without_prefix and nick_at helpers

`IRCChannelMemberListModel->nick_at` returns the nick name of a channel
member at the specified index.

`IRCClient->nick_without_prefix` returns a formatted nick name without
privilege prefix.

Applications/IRCClient/IRCChannelMemberListModel.cpp

@@ -81,3 +81,8 @@ void IRCChannelMemberListModel::update()
 {
     did_update();
 }
+
+String IRCChannelMemberListModel::nick_at(const GUI::ModelIndex& index) const
+{
+    return data(index, IRCChannelMemberListModel::Role::Display).to_string();
+}