FEATURE: Watched words improvements (#7899)

This commit contains 3 features:

- FEATURE: Allow downloading watched words
This introduces a button that allows admins to download watched words per action in a `.txt` file.

- FEATURE: Allow clearing watched words in bulk
This adds a "Clear All" button that clears all deleted words per action (e.g. block, flag etc.)

- FEATURE: List all blocked words contained in the post when it's blocked
When a post is rejected because it contains one or more blocked words, the error message now lists all the blocked words contained in the post.

-------

This also changes the format of the file for importing watched words from `.csv` to `.txt` so it becomes inconsistent with the extension of the file when watched words are exported.

spec/integration/watched_words_spec.rb

@@ -13,6 +13,7 @@ describe WatchedWord do
   let(:require_approval_word) { Fabricate(:watched_word, action: WatchedWord.actions[:require_approval]) }
   let(:flag_word) { Fabricate(:watched_word, action: WatchedWord.actions[:flag]) }
   let(:block_word) { Fabricate(:watched_word, action: WatchedWord.actions[:block]) }
+  let(:another_block_word) { Fabricate(:watched_word, action: WatchedWord.actions[:block]) }
 
   before_all do
     WordWatcher.clear_cache!
@@ -27,7 +28,7 @@ describe WatchedWord do
       expect {
         result = manager.perform
         expect(result).to_not be_success
-        expect(result.errors[:base]&.first).to eq(I18n.t('contains_blocked_words', word: block_word.word))
+        expect(result.errors[:base]&.first).to eq(I18n.t('contains_blocked_word', word: block_word.word))
       }.to_not change { Post.count }
     end
 
@@ -51,6 +52,15 @@ describe WatchedWord do
       should_block_post(manager)
     end
 
+    it "should block the post if it contains multiple blocked words" do
+      manager = NewPostManager.new(moderator, raw: "Want some #{block_word.word} #{another_block_word.word} for cheap?", topic_id: topic.id)
+      expect {
+        result = manager.perform
+        expect(result).to_not be_success
+        expect(result.errors[:base]&.first).to eq(I18n.t('contains_blocked_words', words: [block_word.word, another_block_word.word].join(', ')))
+      }.to_not change { Post.count }
+    end
+
     it "should block in a private message too" do
       manager = NewPostManager.new(
         tl2_user,