Extract user email from authentication token, use configuration property to define displayed user name.

Fix # When using oauth2 or optional_oauth2 random UUID is displayed instead of user email or full name.

Describe changes proposed in this pull request:

• for oauth2 authentication extract email from Principal class
• use configuration property skin.user_display_name to define what value should be show for user name, this property was already defined but never used

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• The commit log is comprehensible. It follows 7 rules of great commit messages. We can fix this during merge by using a squash+merge if necessary
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• Is this PR adding logic based on one or more clinical attributes? If yes, please make sure validation for this attribute is also present in the data validation / data loading layers (in backend repo) and documented in File-Formats Clinical data section!
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