v2. Make "normal"-face hash table local to server buffers and double as
cache for inserted `nicks' faces. "Pre-combine" generated `nicks' faces
with `erc-nicks-backing-face' via :include. Overload
`erc-button-add-button' NICKP param (internally) for conveying current
`erc-button--nick' object. Remove `match'-based combo faces from
`erc-track-faces-priority-list' and `erc-track-faces-normal-list'.
Change default of `erc-button-nickname-face' to new face for
distinguishing between button-applied and "speaker" faces.