Maintain case-insensitivity when merging multiple matchers (#213)

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.

app/models/glitch/keyword_mute.rb

@@ -35,7 +35,7 @@ class Glitch::KeywordMute < ApplicationRecord
     def initialize(account_id)
       @account_id = account_id
       regex_text = Rails.cache.fetch("keyword_mutes:regex:#{account_id}") { regex_text_for_account }
-      @regex = /#{regex_text}/i
+      @regex = /#{regex_text}/
     end
 
     def =~(str)
@@ -60,7 +60,7 @@ class Glitch::KeywordMute < ApplicationRecord
       sb = keyword =~ /\A[[:word:]]/ ? '\b' : ''
       eb = keyword =~ /[[:word:]]\Z/ ? '\b' : ''
 
-      /#{sb}#{Regexp.escape(keyword)}#{eb}/
+      /(?mix:#{sb}#{Regexp.escape(keyword)}#{eb})/
     end
   end
 end