Make erc-get-user-mode-prefix more flexible

Make erc-get-user-mode-prefix more flexible

* etc/ERC-NEWS: Mention renaming of `erc-get-user-mode-prefix'.
* lisp/erc/erc-speedbar.el (erc-speedbar-insert-user): Use
`erc-get-channel-membership-prefix' so that nicks in the nickbar can
have prefixes beyond just those for "voice" and "op".
* lisp/erc/erc.el (erc-get-user-mode-prefix,
erc-get-channel-membership-prefix): Rename former to latter because
"user mode" suggests the function somehow involves user modes, but it
exclusively concerns channel modes.  Also, overload the only parameter
in order to avoid redundantly looking up `erc-channel-user' object
with every predicate call.  In the near future, ERC will likely need
to offer an alternate version of this function that returns multiple
prefixes instead of just one.
(erc-format-@nick): Actually use the `channel-data' parameter.
(erc-format-my-nick, erc--format-channel-status-prefix): Use new name
for function `erc-get-user-mode-prefix'.  (Bug#63595)  (Bug#67677)