Refactor Credential Handling

[ogoffart]

We have had a lot of regression in the handling of credentials. Issues with keychain, logins log off, shibboleth, interaction with the wizard, ...
We also want to support in the future multiple accounts and UI-less library for the command line client.
The code has become more complicated than it should be and some refactoring is needed.

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[ogoffart]

I think we probably should not have AbstractCrediential. Just handle the 401 or the redirection when they come.

[danimo]

How is one related to the other?

[guruz]

Also related to #43

[guruz]

@ogoffart @ckamm is this (partly?) done?

[ckamm]

Some of it is done, some if it might want to go into more specific tickets @ogoffart.