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.
