Avoid using sprockets for combining stylesheets

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