Use regular spaces, rather than non-breaking spaces, in formatted filenames

[rebeccacremona]

This tweaks the output of two fields in our report about how things went, when converting a WARC to a WACZ.

The difference:

In [1]: from django.template.defaultfilters import filesizeformat

In [2]: filesizeformat(100)
Out[2]: '100\xa0bytes'

In [3]: filesizeformat(100).replace("\xa0", " ")
Out[3]: '100 bytes'

See ENG-923.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 69.50%. Comparing base (7d556e4) to head (902671c). Report is 75 commits behind head on develop.

:exclamation: Current head 902671c differs from pull request most recent head b20e91f

Please upload reports for the commit b20e91f to get more accurate results.

Files	Patch %	Lines
perma_web/perma/celery_tasks.py	0.00%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3534 +/- ## =========================================== - Coverage 69.53% 69.50% -0.04% =========================================== Files 48 48 Lines 6785 6787 +2 =========================================== - Hits 4718 4717 -1 - Misses 2067 2070 +3 ```

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