fix search fields for djago admin

[kimakan]

Problem: the search for some models (e.g., DownloadJobs) in Django Admin didn't work (internal server error).

Bug: the search fields didn't match the fields of the corresponding model. In some models __str__ was used as a search field which doesn't seem to work.

Fix: changed the search_field accordingly to match the corresponding model

[coveralls]

Pull Request Test Coverage Report for Build 3600793026

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.883%

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Totals
Change from base Build 3513124880:	0.0%
Covered Lines:	4598
Relevant Lines:	6979

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💛 - Coveralls

[agy-why]

closes #52