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).

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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