Raise 404 at SubdomainMiddleware if the project does not exist.

[dojutsu-user]

Fixes #3801

[codecov]

Codecov Report

Merging #4795 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4795      +/-   ##
==========================================
+ Coverage    75.3%   75.31%   +<.01%     
==========================================
  Files         158      158              
  Lines       10047    10049       +2     
  Branches     1267     1268       +1     
==========================================
+ Hits         7566     7568       +2     
  Misses       2145     2145              
  Partials      336      336

Impacted Files	Coverage Δ
readthedocs/core/middleware.py	84.55% <100%> (+0.25%)	⬆️

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[dojutsu-user]

@humitos I have implemented this with the .exists() as discussed in the comment #3801 (comment) and also added the tests for code coverage.
Please review.

[ericholscher]

Makes sense. This should only really be happening on 404's already (since docs will be served directly from nginx if they exist) -- but seems like a useful sanity check.