Switch to sassc-rails

Closes #18432

This uses sassc to compile the application's SASS so it compiles faster.

Without attempting to be very scientific here are some numbers I got:

Using sassc-rails:

```
[1] pry(main)> Benchmark.bm { |bm| bm.report { Rails.application.assets["application.css"] } }
       user     system      total        real
   1.430000   0.380000   1.810000 (  1.830753)
```

Using sass-rails:
```
[1] pry(main)> Benchmark.bm { |bm| bm.report { Rails.application.assets["application.css"] } }
       user     system      total        real
  12.320000   0.530000  12.850000 ( 12.909684)
```

The result is faster page loads when changing CSS in development and
faster precompilation.

Gemfile

@@ -219,7 +219,7 @@ gem 'oj', '~> 2.17.4'
 gem 'chronic', '~> 0.10.2'
 gem 'chronic_duration', '~> 0.10.6'
 
-gem 'sass-rails', '~> 5.0.6'
+gem 'sassc-rails', '~> 1.3.0'
 gem 'coffee-rails', '~> 4.1.0'
 gem 'uglifier', '~> 2.7.2'
 gem 'gitlab-turbolinks-classic', '~> 2.5', '>= 2.5.6'

Gemfile.lock

@@ -675,12 +675,17 @@ GEM
     sanitize (2.1.0)
       nokogiri (>= 1.4.4)
     sass (3.4.22)
-    sass-rails (5.0.6)
-      railties (>= 4.0.0, < 6)
-      sass (~> 3.1)
-      sprockets (>= 2.8, < 4.0)
-      sprockets-rails (>= 2.0, < 4.0)
-      tilt (>= 1.1, < 3)
+    sassc (1.11.1)
+      bundler
+      ffi (~> 1.9.6)
+      sass (>= 3.3.0)
+    sassc-rails (1.3.0)
+      railties (>= 4.0.0)
+      sass
+      sassc (~> 1.9)
+      sprockets (> 2.11)
+      sprockets-rails
+      tilt
     sawyer (0.8.1)
       addressable (>= 2.3.5, < 2.6)
       faraday (~> 0.8, < 1.0)
@@ -980,7 +985,7 @@ DEPENDENCIES
   ruby-prof (~> 0.16.2)
   rugged (~> 0.24.0)
   sanitize (~> 2.0)
-  sass-rails (~> 5.0.6)
+  sassc-rails (~> 1.3.0)
   scss_lint (~> 0.47.0)
   seed-fu (~> 2.3.5)
   select2-rails (~> 3.5.9)

changelogs/unreleased/switch-to-sassc.yml

+---
+title: Switch to sassc-rails for faster stylesheet compilation
+merge_request: 8556
+author: Richard Macklin