Add Scalar UI support as a third documentation option

[huynguyengl99]

Implement #1426

Adds Scalar as a third documentation UI option alongside SwaggerUI and ReDoc.

Usage:

from drf_spectacular.views import SpectacularScalarView

urlpatterns = [
    path('api/schema/scalar/', SpectacularScalarView.as_view(url_name='schema'), name='scalar'),
]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (e71903e) to head (246c4e7).
⚠️ Report is 79 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1425   +/-   ##
=======================================
  Coverage   98.58%   98.59%           
=======================================
  Files          76       76           
  Lines        9194     9225   +31     
=======================================
+ Hits         9064     9095   +31     
  Misses        130      130           

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[JonWoodlief]

I am interested in this

[huynguyengl99]

I hope @tfranzel could have some time to review this PR and consider merging it as well, as Scalar documentation is quite good IMO and the UI is very nice, but looks like he is quite busy now to take a look :(