[MH-13034] Revert use to standardized LTI field instead of Moodle specific field for the username - Opencast

Details

Severity:
Incorrectly Functioning Without Workaround
Steps to reproduce:

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Introduced by ~~MH-12840~~:

The Moodle-only field `ext_user_username` is used to extract the the username form the LTI authentication request instead of the optional standardized field `lis_person_sourcedid`.

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Introduced by MH-12840 : The Moodle-only field `ext_user_username` is used to extract the the username form the LTI authentication request instead of the optional standardized field `lis_person_sourcedid`.