Spaces add plant

[Danielgergely]

• Added service methods to add/remove plants to/from spaces
• Added controller methods to add/remove plants to/from spaces
• Fixed delete method in service: deleted space is removed from plants as well
• Fixed a type & removed unused imports

[M-Sigg]

• Adjusted the tests according to the changes.
• Added controller tests for added endpoints.
• Functionality for adding/deleting plants to a space was already implemented in the plantController and plantService, if those endpoints and methods aren't used by the frontend, we should delete them for less code duplication (and move the service tests for adding/ deleting plants to/from space to the space service test files)
• Otherwise this can be merged imo

[Danielgergely]

• just added a check for space names. please also check

[M-Sigg]

check looks good, had to adjust a test