Don't add \b to whole-word keywords that don't start with word characters.

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.

spec/models/glitch/keyword_mute_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Glitch::KeywordMute, type: :model do
   describe '.matcher_for' do
     let(:matcher) { Glitch::KeywordMute.matcher_for(alice) }
 
-    describe 'with no Glitch::KeywordMutes for an account' do
+    describe 'with no mutes' do
       before do
         Glitch::KeywordMute.delete_all
       end
@@ -17,7 +17,7 @@ RSpec.describe Glitch::KeywordMute, type: :model do
       end
     end
 
-    describe 'with Glitch::KeywordMutes for an account' do
+    describe 'with mutes' do
       it 'does not match keywords set by a different account' do
         Glitch::KeywordMute.create!(account: bob, keyword: 'take')
 
@@ -63,7 +63,13 @@ RSpec.describe Glitch::KeywordMute, type: :model do
       it 'matches keywords surrounded by non-alphanumeric ornamentation' do
         Glitch::KeywordMute.create!(account: alice, keyword: 'hot')
 
-        expect(matcher =~ 'This is a ~*HOT*~ take').to be_truthy
+        expect(matcher =~ '(hot take)').to be_truthy
+      end
+
+      it 'escapes metacharacters in keywords' do
+        Glitch::KeywordMute.create!(account: alice, keyword: '(hot take)')
+
+        expect(matcher =~ '(hot take)').to be_truthy
       end
 
       it 'uses case-folding rules appropriate for more than just English' do