Cover all auth providers with tests, clean up env

[paskal]

Add all auth providers to tests for consistency, reset all variables containing sensitive data on remark42 start (a couple were missing). Changes were done in #1022, but I thought it makes sense to merge them separately.

[coveralls]

Pull Request Test Coverage Report for Build 872358470

• 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.09%) to 83.663%

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Files with Coverage Reduction	New Missed Lines	%
backend/app/notify/notify.go	2	94.29%

Totals
Change from base Build 872139555:	0.09%
Covered Lines:	5413
Relevant Lines:	6470

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💛 - Coveralls