[QA] username and password fields are not forwarded to kopano IDM

seen while testing 2.1.1 - but not a regression, same with earlier versions

• admin enters his username and password in the owncloud login page.
• admin hits the 'Login' button, login succeeds. OK

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• ldap user enters his username and password in the owncloud login page.
• ldap user hits the 'Kopano' button, his credentials are wiped out. BAD.
• he has to enter them again. then the login succeeds via IDP.

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Expected behaviour:

• UX-background: As both buttons 'Login' and 'Kopano' are styled identical and one is advertised as an alternative to the other, it is reasonable that users assume the actions on this page are identical in both cases.
• The kopano login expects that the login data are only entered after hitting the 'Kopano' button, but not before. In case the user has entered them before, they should be pre-filled into the IDP-login page.