✨ (bootsier): Añade AdminLTE 4 y Bootstrap Icons

Integra AdminLTE 4.0.0 (SCSS y JS), Bootstrap Icons 1.13.1 y las fuentes
Source Sans 3 VF; reorganiza los SCSS del tema con la nueva estructura
de *pagetop-build*.

extensions/pagetop-bootsier/static/scss/bootstrap-5.3.8/mixins/_image.scss

@@ -1,16 +0,0 @@
-// Image Mixins
-// - Responsive image
-// - Retina image
-
-
-// Responsive image
-//
-// Keep images from scaling beyond the width of their parents.
-
-@mixin img-fluid {
-  // Part 1: Set a maximum relative to the parent
-  max-width: 100%;
-  // Part 2: Override the height to auto, otherwise images will be stretched
-  // when setting a width and height attribute on the img element.
-  height: auto;
-}