Use helper to get erc-networks--id symbol as string

Use helper to get erc-networks--id symbol as string

* lisp/erc/erc-networks.el (erc-networks--id-string): New function.
(erc-networks--construct-target-buffer-name)
(erc-networks--ensure-unique-server-buffer-name)
(erc-networks--rename-server-buffer): Use it.
* lisp/erc/erc-nicks.el (erc-nicks-list-faces): Use it.
* lisp/erc/erc-status-sidebar.el (erc-status-sidebar-pad-hierarchy): Use it.
* lisp/erc/erc.el (erc-generate-new-buffer-name)
(erc-log-irc-protocol)
(erc--auth-source-determine-params-defaults)
(erc-format-target-and/or-network): Use it.
* test/lisp/erc/erc-networks-tests.el (erc-networks--id-string): New test.

(cherry picked from commit 7fc048ca7781f9ba04d953f79fb29646c92cf35d)