Resolve some byte-compiler warnings, use new ERC buffer switching functions

* lisp/bandali-erc.el: Sprinkle a few `declare-function's here and
there, and use brand new functions `erc-switch-to-buffer-other-window'
and `erc-track-switch-buffer-other-window' I just added to emacs-27 in
38f7538d8f62ee287e8271d048f1230d840c11a0 and
13301d4266d26882f9fe7efe3046accd315d7c55 of emacs.git respectively.