FIX: Split link watched words from replace (#13196)

It was not clear that replace watched words can be used to replace text
with URLs. This introduces a new watched word type that makes it easier
to understand.

db/migrate/20210528144647_migrate_watched_words_from_replace_to_link.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class MigrateWatchedWordsFromReplaceToLink < ActiveRecord::Migration[6.1]
+  def up
+    execute <<~SQL
+      UPDATE watched_words
+      SET action = 8
+      WHERE action = 5 AND replacement ILIKE 'http%'
+    SQL
+  end
+
+  def down
+    execute("UPDATE watched_words SET action = 5 WHERE action = 8")
+  end
+end