holoirc: rework PMs completely and Handlers paritally

PMs:
* split User into 2 classes - ChannelUser and PrivateMessageUser
* only store users who are present in channels in the UCI
* only store PM users in the Server list
* also make all variants of NickServ and ChanServ the same (i.e. all variations in capitalisation)

Handlers:
* only register callback interfaces in the Server on the IRC side and the Activity on the Fragment side
* retrieve all the handlers through this callback interface
* remove all the registration and deregisteration from channels and users
* modify the IRCPagerAdapter to check the type as well as the name to make things less likely to fail