Fix uploading assets on windows machines

I was running into errors on my windows machine, when uploading assets using create_all


      File "C:\Users\Franklyn\Envs\comanage\lib\site-packages\flask_s3.py", line 118, in _write_files
    asset_loc)
      File "C:\Users\Franklyn\Envs\comanage\lib\site-packages\flask_s3.py", line 105, in _static_folder_path
    (static_asset, static_folder))
    ValueError: /Users/Franklyn/Source/comanage/comanage/public/.webassets-cache/035ed7663649a13ac64ecdeb9860a493 static asset must be under C:\Users\Franklyn\Source\comanage\comanage\public static folder

This was due to the _path_to_relative_url converting C:\Users\Franklyn\Source\comanage\comanage\public to  /Users/Franklyn/Source/comanage/comanage/public/ but then attempting to compare to the original string. 

This PR will use the same relative_url function to provide equivalent comparisons for these paths.

I was unable to write a test for this case, as it did not exhibit the same behavior on other systems