Authentication: Different return values between base-token endpoints

[christophdb]

The "Get base-token with API-token" endpoint returns 8 values while "Get base-token with account-token" returns only 5 values. It should be clarified why these endpoints return different sets of fields and whether they should be harmonized.

[christophdb]

Added as API issue #25 in API_ISSUES.md (commit 66b44bb). Documented the field differences between the two base-token endpoints.