Refactor data structure to introduce Summary Functionality

[yanik-recke]

See sub-issues.

MetadataService endpoints:

1. GetModules - Lists all existing modules, contains the metadata to all the lectures for that module
2. DeleteModule - Deletes an existing module
3. DeleteLecture - Deletes an existing lecture
4. AddLecture - Adds a lecture to a module (including the metadata)

Changes would result in the following architecture overview: