Moving middleware user auth check to set_actor()

[ndwhelan]

Move the check for whether the user is authenticated from before
registering set_actor() as the pre_save signal receiver
in to set_actor(). This enables two things:

1. remote_addr in the LogEntry is set even if the user is
   not authenticated.
2. This middleware no longer execute before the user is set
   on the request. Some frameworks may set this in a
   following middleware. Some frameworks, such as the widely
   adopeted Django Rest Framework do not set this as part
   of a middleware. So, the upshot of hit is that the
   actor will not be set when using Django Rest Framework.

[ndwhelan]

Addresses #277

[codecov]

Codecov Report

Merging #279 into master will increase coverage by 1.84%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage   82.20%   84.04%   +1.84%     
==========================================
  Files          18       18              
  Lines         489      489              
==========================================
+ Hits          402      411       +9     
+ Misses         87       78       -9     

Impacted Files	Coverage Δ
auditlog/middleware.py	92.10% <100.00%> (+23.68%)	⬆️

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[ndwhelan]

This is probably solved by PR #262

[aleh-rymasheuski]

This is actually not solved in #262. #262 sets both actor and remote_addr only for authenticated users, which is exactly the issue attempted to be resolved by this pull request.

[aqeelat]

Superseded by #484
@hramezani

[hramezani]

Thanks @ndwhelan for the initial patch and @aqeelat for continuing on this.