Fill out some examples for Glitch::FilterHelper. #234.

Also add HTML entity decoding to Glitch::FilterHelper, which is needed
to e.g. match "<" to the tag-stripped version of "<p><3</p>" or
"<p>&lt;3</p>".

spec/models/glitch/filter_helper_spec.rb

@@ -0,0 +1,43 @@
+require 'rails_helper'
+
+RSpec.describe Glitch::FilterHelper do
+  describe '#matches?' do
+    let(:alice) { Fabricate(:account, username: 'alice').tap(&:save!) }
+    let(:helper) { Glitch::FilterHelper.new(alice) }
+
+    it 'ignores names of HTML tags in status text' do
+      status = Fabricate(:status, text: '<addr>uh example</addr>')
+      Glitch::KeywordMute.create!(account: alice, keyword: 'addr')
+
+      expect(helper.matches?(status)).to be false
+    end
+
+    it 'ignores properties of HTML tags in status text' do
+      status = Fabricate(:status, text: '<a href="https://www.example.org">uh example</a>')
+      Glitch::KeywordMute.create!(account: alice, keyword: 'href')
+
+      expect(helper.matches?(status)).to be false
+    end
+
+    it 'matches text inside HTML tags' do
+      status = Fabricate(:status, text: '<p>HEY THIS IS SOMETHING ANNOYING</p>')
+      Glitch::KeywordMute.create!(account: alice, keyword: 'annoying')
+
+      expect(helper.matches?(status)).to be true
+    end
+
+    it 'matches < in HTML-stripped text' do
+      status = Fabricate(:status, text: '<p>I <3 oats</p>')
+      Glitch::KeywordMute.create!(account: alice, keyword: '<3')
+
+      expect(helper.matches?(status)).to be true
+    end
+
+    it 'matches &lt; in HTML text' do
+      status = Fabricate(:status, text: '<p>I &lt;3 oats</p>')
+      Glitch::KeywordMute.create!(account: alice, keyword: '<3')
+
+      expect(helper.matches?(status)).to be true
+    end
+  end
+end