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.

1 files changed, 1 insertions, 1 deletions

diff --git a/Gemfile b/Gemfile
index 07ff500dfea..62e43b798f7 100644
--- a/Gemfile
+++ b/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'