Tips: Allow creation by teachers

[MrSerth]

As of today, only administrators are allowed to add new tips. Teachers may then choose existing tips to add to their exercise. This process works, but isn't really encouraging for teachers to add new tips. Therefore, we would like to change the current process and allow teachers to add new tips.

When enabling teachers to create new tips, we should ideally consider potential duplicated tips; maybe by showing existing ones or some other de-duplication mechanism.