Add nickserv certfp authentication

[Svetlana-T]

https://freenode.net/kb/answer/certfp

https://www.oftc.net/NickServ/CertFP/

Would be nice to have, as some channels require authentication prior to joining and for the lack of a working automatic identification option, currently I have to join them by hand.

[Wikiwide]

Would be nice. Especially taking into account that I regularly see disconnections (for whatever reasons), and client, annoyingly, is unable to automatically rejoin the channel.

[jhthorsen]

Does that mean we should have two new fields in the connection settings, where you can upload a private and public key? If, so I think the building blocks are in place:

• 69213a6 has logic for uploading files that should only be accessible from the server.
• 26bbf62 has logic for reading tls_cert and tls_key, which could come from the files uploaded.