Migrate all admin pages to use controllers

[thorsten]

• backup.main.php
• category.showstructure.php
• elasticsearch.php
• forms.php
• header.php
• instances.php
• open-questions.php
• statistics.ratings.php
• stickyfaqs.php
• upgrade.php
• category.add.php
• category.translate.php
• export.php
• forms.translations.php
• login.php
• password.change.php
• statistics.search.php
• stopwords.php
• user.php
• attachments.php
• category.edit.php
• comments.php
• faqs.editor.php
• import.csv.php
• media.browser.php
• report.main.php
• statistics.sessions.day.php
• system.php
• backup.export.php
• category.main.php
• configuration.php
• faqs.overview.php
• glossary.php
• index.php
• news.php
• session.keepalive.php
• statistics.sessions.php
• tags.php
• backup.import.php
• category.overview.php
• dashboard.php
• footer.php
• group.php
• instances.edit.php
• no-permission.php
• statistics.admin-log.php
• statistics.show.php
• twofactor.php