FIX: Restore trust level when leaving group (#17954)

If a user was granted a trust level, joined a group that granted a trust
level and left the group, the trust level was reset. This commit tries
to restore the last known trust level before joining the group by
looking into staff logs.

This commit also migrates old :change_trust_level user history records
to use previous_value and new_value fields.

db/migrate/20220818171849_move_tl_user_history_to_previous_and_new_value.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class MoveTlUserHistoryToPreviousAndNewValue < ActiveRecord::Migration[7.0]
+  def change
+    execute <<~SQL
+      UPDATE user_histories
+      SET previous_value = old_tl,
+          new_value = new_tl,
+          details = NULL
+      FROM (
+          SELECT id user_history_id,
+                 (REGEXP_MATCHES(details, 'old trust level: (\d+)', 'i'))[1] old_tl,
+                 (REGEXP_MATCHES(details, 'new trust level: (\d+)', 'i'))[1] new_tl
+          FROM user_histories
+          WHERE action = 2
+      ) trust_levels
+      WHERE user_histories.id = trust_levels.user_history_id
+    SQL
+  end
+end