Students: add API to register student to an emphasisline, and to retrieve that information

[Daníel Sigurgeirsson]

We should add a PUT method which either registers a student (for a given registration) on a given emphasisline, or updates the current line registration. Note: we should keep a version history of all changes.

Implementation details: * add two fields to the Student_Registrations table, with EmphasisLineID (default: null), and CurriculumID (default: null). Add a new table: _StudentEmphasisLineRegistrationHistory, which contains a log of all changes to the first table.

[Daníel Sigurgeirsson]

Finished in PR #185