Quality-time stops contacting Jenkins due to Jenkins wrongly returning a 401

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The Jenkins LDAP-plugin sometimes fails to connect to the LDAP-server causing Jenkins to wrongly return a 401. Quality-time then assumes the user has entered a wrong password and stops contacting the source.

A work-around could be to only stop contacting sources when the credentials are new and not when we know they've been working in the past. And/or keep trying for a few times when we get a 401.

Considered but dismissed options for a work-around:

• Work-around cannot be made specific for the Jenkins source, because Jenkins can also function as webserver for other sources such as JUnit, JaCoCo, and OJAudit reports.
• Jira returns a 401 when submitting an invalid password, so simply distinguishing between 401 and 403 won't be a work-around either.

See https://issues.jenkins-ci.org/browse/JENKINS-54015.

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Branch: https://github.com/ICTU/quality-time/tree/issue-652-jenkins-401

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We decided on removing the functionality to stop contacting sources after a 401 or 403. The risk that Quality-time will lock accounts after users enter a wrong password should be signficantly smaller due to #478.