Add custom storage backend for GCS

[np5]

This is a workaround, to be able to sign blob URLs without a credential
file/private key.

References:

jschneier/django-storages#941
https://googleapis.dev/python/storage/latest/blobs.html#google.cloud.storage.blob.Blob.generate_signed_url

[coveralls]

Pull Request Test Coverage Report for Build 2999450970

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 32 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 65.554%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zentral/utils/storage.py	0	1	0.0%
zentral/utils/gcs_storage.py	0	31	0.0%

Totals
Change from base Build 2894883779:	-0.3%
Covered Lines:	24179
Relevant Lines:	36884

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💛 - Coveralls