Sideline implied invisible-intangible coupling in ERC

Sideline implied invisible-intangible coupling in ERC

* etc/ERC-NEWS: Add entry explaining removal of automatic `intangible'
propertizing of t-valued `invisible' messages.
* lisp/erc/erc.el (erc--insert-invisible-as-intangible-p): New flag
variable, a temporary escape hatch to regain pre-5.6 behavior
involving the modification of certain `invisible' messages.
(erc--insert-line): Gate unfavorable behavior behind
`erc--insert-invisible-as-intangible-p' flag.  Add comment clarifying
deferred initialization of `insert-position', which was part of the
many changes introduced as part of bug#60936.

(cherry picked from commit 775bd4b631e9303c20e4ebddd179960276065448)