Different CSS properties for .historic

On Mountain Lion and Mavericks, opacity is used
On Yosemite and later, -webkit-filter is used

This is a result of reported scrolling lag on Mavericks and earlier as
a result of using -webkit-filter
Fix incorrect default popup selection
Add /autojoin/ command

Calling the /autojoin/ command will manually invoke auto join on
networks that do not have NickServ but are configured to wait for
responses from it.

This adds convenience when the command is added to the list of connect
commands.

The command itself will only perform auto join if it is not already in
progress or already happened.