common: Port HighlightRule to ExpressionMatch

Port HighlightRule objects to ExpressionMatch class, providing easy
caching and simplifying expression handling.

Migrate HighlightRule struct into a full-blown class for easier
management and greater assurance over automatic internal cache
management.

Port HighlightRuleManager to ExpressionMatch for nickname matching,
providing easy caching and simplifying expression handling.
(Noticing a theme?)

Add tons of documentation comments, too, and fix up line lengths.