minor #1450 Remove Sass from custom CSS files (javiereguiluz)

This PR was squashed before being merged into the main branch.

Discussion
----------

Remove Sass from custom CSS files

This is mentioned by `@weaverryan` in #1449.

We don't really use any Sass feature (except nesting and some variables). I've replaced Sass variables by CSS custom properties ... and removed the nesting by flattening all styles (we can readd nesting soon when native support is a big bigger than today: https://caniuse.com/?search=nesting).

**Ryan**, if you agree on this, can we just merge it and then you rebase your PR? Thanks!

Commits
-------

691859e Remove Sass from custom CSS files

Author: javiereguiluz

assets/styles/admin.scss

@@ -2,6 +2,10 @@
 // @import "~eonasdan-bootstrap-datetimepicker/src/sass/bootstrap-datetimepicker-build.scss";
 @import "bootstrap-tagsinput.scss";
 
+:root {
+  --blue: #007bff;
+}
+
 /* Page: 'Backend post index'
    ------------------------------------------------------------------------- */
 body#admin_post_index .item-actions {
@@ -27,9 +31,8 @@ body#admin_post_show .post-tags .label-default i {
 
 .form-control {
   border-width: .125rem;
-
-  &:focus {
-    color: $blue;
-    box-shadow: none;
-  }
-}
+}
+.form-control:focus {
+  color: var(--blue);
+  box-shadow: none;
+}