Upgrade NSS to 3.15.4
Add missing Entrust.net certificate for AIM
Add Ethan's AIM cert fix to ChangeLog.
Correctly send empty IRC SASL responses. Fixes #15452
Add Mantas Mikul?nas to COPYRIGHT. Refs #15452
Change IRC Username label to Ident name

This field should really be the UNIX login name on the local machine and
is used in the Ident protocol. It also shows up in the users hostmask.

Hopefully this will avoid confusion with the "Username" field on the
general tab, which is really the preferred IRC nickname.

Refs #15451
Tidy up our local certificate store. Fixes #15850