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

js/dark-mode-switch.js

@@ -0,0 +1,50 @@
+/*!
+ * Color mode toggler for Bootstrap's docs (https://getbootstrap.com/)
+ * Copyright 2011-2024 The Bootstrap Authors
+ * Licensed under the Creative Commons Attribution 3.0 Unported License.
+ * Modified to work with a simpler checkbox toggle
+ */
+
+(() => {
+  'use strict'
+
+  const getStoredTheme = () => localStorage.getItem('theme')
+  const setStoredTheme = theme => localStorage.setItem('theme', theme)
+  const getPreferredTheme = () => window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+
+  const getStoredPreferredTheme = () => {
+    const storedTheme = getStoredTheme()
+    if (storedTheme) {
+      return storedTheme
+    }
+    return getPreferredTheme()
+  }
+
+  const setTheme = theme => {
+    if (theme === 'auto') {
+      document.documentElement.setAttribute('data-bs-theme', getPreferredTheme())
+    } else {
+      document.documentElement.setAttribute('data-bs-theme', theme)
+    }
+  }
+
+  setTheme(getStoredPreferredTheme())
+
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', () => {
+    const storedTheme = getStoredTheme()
+    if (storedTheme !== 'light' && storedTheme !== 'dark') {
+      setTheme(getStoredPreferredTheme())
+    }
+  })
+
+  window.addEventListener('DOMContentLoaded', () => {
+    const toggle = document.querySelector('#bd-theme')
+    toggle.checked = getStoredTheme() === 'dark'
+    toggle.addEventListener('change', (event) => {
+      const theme = event.currentTarget.checked ? 'dark' : 'light'
+      setStoredTheme(theme)
+      setTheme(theme)
+      event.currentTarget.focus()
+    })
+  })
+})()