=~ made sense when we were passing it through to a regex, but we're no
longer doing that: TagMatcher looks at individual tags and returns a
value that *looks* like what you get out of #=~ but really isn't that
meaningful. Probably a good idea to not subvert convention like this
and instead use a name with guessable intent.
It is reasonable to expect someone to enter #foo to mute hashtag #foo.
However, tags are recorded on statuses without the preceding #.
To adjust for this, we build a separate tag matcher and use
Tag::HASHTAG_RE to extract a hashtag from the hashtag syntax.
When given two regexps, Regexp.union preserves the options set (or not
set) on each regex; this meant that none of the multiline (m),
case-insensitivity (i), or extended syntax (x) options were set. Our
regexps are written expecting the m, i, and x options were set on all of
them, so we need to make sure that we preserve that behavior.
Ditto for ending with \b.
Consider muting the phrase "(hot take)". I stipulate it is reasonable
to enter this with the default "match whole word" behavior. Under the
old behavior, this would be encoded as
\b\(hot\ take\)\b
However, if \b is before the first character in the string and the first
character in the string is not a word character, then the match will
fail. Ditto for after. In our example, "(" is not a word character, so
this will not match statuses containing "(hot take)", and that's a very
surprising behavior.
To address this, we only add leading and trailing \b to keywords that
start or end with word characters.
There are two motivations for this:
1. It looks like we're going to add other features that require
server-side storage (e.g. user notes).
2. Namespacing glitchsoc modifications is a good idea anyway: even if we
do not end up doing (1), if upstream introduces a keyword-mute feature
that also uses a "KeywordMute" model, we can avoid some merge
conflicts this way and work on the more interesting task of
choosing which implementation to use.
