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.

app/controllers/export_csv_controller.rb

@@ -12,6 +12,7 @@ class ExportCsvController < ApplicationController
   end
 
   private
+
   def export_params
     @_export_params ||= begin
       params.require(:entity)