Rework API level editing, refactor icon validation

[Toastbrot236]

• The normal and the admin API endpoint for editing a level now use separate request body classes/interfaces and DB methods
• These DB methods no longer use reflection. They now update levels similarly to how most other similar methods do it now (handle every wanted attribute separately). Closes Dont use reflection to update level details #579
• All API endpoints which need to validate icons now use one helper method for this. Removes duplicate code and inconsistent validation processes this way.