initial work on a bootstrap 5 template

current status:
- renders without PHP errors & passes unit tests
- displays pastes
- responsive navbar
- right-to-left support
- auto dark mode with toggle

to be done:
- add "Dark Mode" to translation strings
- get expiration and format selections to work
- fix modals (password, QR-code, etc.)
- replace glyphicons with Bootstrap Icons (no longer included)
- test all the different settings and combinations
- check tab alignment in HTML source

css/bootstrap5/privatebin.css

@@ -0,0 +1,12 @@
+/**
+ * PrivateBin
+ *
+ * a zero-knowledge paste bin
+ *
+ * @link      https://github.com/PrivateBin/PrivateBin
+ * @copyright 2012 Sébastien SAUVAGE (sebsauvage.net)
+ * @license   https://www.opensource.org/licenses/zlib-license.php The zlib/libpng License
+ * @version   1.7.1
+ */
+
+ .hidden { display: none; }