Extend model relationships and validations

[MrSerth]

Currently, the different models are not linked as good as possible. Some lack a respective belongs_to or has_many relationship, which would ease accessing the different related objects. Also, some model validations can be enhanced. We should do that.

Next, some enums (like the cause for submissions and testruns) are missing. So far, we've only added them as enums to Rails (rather than SQL), but even this step is missing.

Furthermore, there might also be some situations where extended validations (like foreign key constraints) can be added to the database. Historically, those were missing.

Once the missing foreign key constrains have been identified, one should clear the production database from invalid entries (in a migration).