Avoid using sprockets for combining stylesheets

There are two options for combining stylesheets, using sprockets
(via `require` statements) or using sass (via `@import` statements).

This commit switches to using `@import` statements throughout, to
avoid a bug with libsass that prevents us from using bootstrap 5.2+

See https://github.com/sass/sassc-rails/issues/174

Since R2 is not triggered when `@import`ing a file, we need to
switch the top-level stylesheet names to include the extension. We
can also take the opportunity to remove a level of indirection,
along with removing symlinks which cause problems on some platforms.

* https://github.com/openstreetmap/openstreetmap-website/issues/3984
* https://github.com/openstreetmap/openstreetmap-website/issues/3422

app/assets/stylesheets/ltr/common.scss

@@ -1 +0,0 @@
-../common.scss

app/assets/stylesheets/print-ltr.css

@@ -1,3 +0,0 @@
-/*
- *= require print
- */

app/assets/stylesheets/print-ltr.scss

@@ -0,0 +1 @@
+@import "print";

app/assets/stylesheets/print-rtl.css

@@ -1,3 +0,0 @@
-/*
- *= require print
- */

app/assets/stylesheets/print-rtl.r2.scss

@@ -0,0 +1 @@
+@import "print";

app/assets/stylesheets/rtl/common.r2.scss

@@ -1 +0,0 @@
-../common.scss

app/assets/stylesheets/screen-ltr.css

@@ -1,3 +0,0 @@
-/*
- *= require ltr/common
- */

app/assets/stylesheets/screen-ltr.scss

@@ -0,0 +1 @@
+@import "common";

app/assets/stylesheets/screen-rtl.css

@@ -1,3 +0,0 @@
-/*
- *= require rtl/common
- */

app/assets/stylesheets/screen-rtl.r2.scss

@@ -0,0 +1 @@
+@import "common";