Redmine

When configuring a webhook for many projects, selecting every project individually is slow. The events section already provides Redmine's standard "Check all | Uncheck all" links, but the project list does not.

This patch adds `check_all_links 'webhook_project_ids'` to the webhook project fieldset, so the existing nested project list can be selected or cleared in one click. It also adds a functional test asserting that both links are rendered.

The patch is against current trunk (r25456 / 7872c0d).

Tests:
- bin/rails test test/functional/webhooks_controller_test.rb
- bundle exec rubocop test/functional/webhooks_controller_test.rb