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.

lib/validators/post_validator.rb

@@ -60,8 +60,15 @@ class Validators::PostValidator < ActiveModel::Validator
   end
 
   def watched_words(post)
-    if !post.acting_user&.staged && matches = WordWatcher.new(post.raw).should_block?
-      post.errors.add(:base, I18n.t('contains_blocked_words', word: matches[0]))
+    if !post.acting_user&.staged && matches = WordWatcher.new(post.raw).should_block?.presence
+      if matches.size == 1
+        key = 'contains_blocked_word'
+        translation_args = { word: matches[0] }
+      else
+        key = 'contains_blocked_words'
+        translation_args = { words: matches.join(', ') }
+      end
+      post.errors.add(:base, I18n.t(key, translation_args))
     end
   end