add a CharField to attachment

[wmles]

for big attachments the user is responsible to upload the file by hand beforehand, then pass the url into this field and save the model. The filefield then links to this attachment

[coveralls]

Pull Request Test Coverage Report for Build 526

• 2 of 6 (33.33%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 69.983%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
scholariumat/products/models.py	2	6	33.33%

Totals
Change from base Build 518:	-0.1%
Covered Lines:	1618
Relevant Lines:	2312

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💛 - Coveralls