Allow updating of /IGNORE timeouts in ERC

Allow updating of /IGNORE timeouts in ERC

* lisp/erc/erc.el (erc--read-time-period, erc--decode-time-period):
Move body of former, now a superficial wrapper, to latter, a new
function.
(erc--format-time-period): New function.
(erc--get-ignore-timer-args): New function.
(erc--find-ignore-timer): New function to search through `timer-list'
to find matching ignore timer.
(erc-cmd-IGNORE): Refactor and redo doc string.  Add new optional
`timespec' parameter, primarily to aid in testing.  Update an existing
timer instead of always creating one, and display time remaining in
"ignore list" output.  Pass server buffer instead of current buffer to
timer callbacks because `erc--unignore-user' displays its messages in
the `active' buffer, not necessarily the issuing one.  Note that doing
this does discard potentially useful information, so if ever reverting,
we can change the `cl-find' :test in `erc--find-ignore-timer' to
something that compares the `erc-server-process' of both buffers.
;;
;; Something like:
;;
;; (defun erc--ignore-timers-equal-p (a b)
;;   (and (equal (car a) (car b))
;;        (eq (buffer-local-value 'erc-server-process (cadr a))
;;            (buffer-local-value 'erc-server-process (cadr b)))))
;;
(erc-cmd-UNIGNORE): Pass `erc-ignore-list' member matching `user'
parameter to `erc--unignore-user' instead of original, raw parameter,
along with the server buffer.
(erc--unignore-user): Cancel existing timer and don't bother switching
to server buffer since we're already there.
(erc-message-english-ignore-list): New variable.
* test/lisp/erc/erc-scenarios-ignore.el: New file.
* test/lisp/erc/erc-tests.el (erc--read-time-period): New test.
(erc-cmd-UNIGNORE): New test.  (Bug#70127)