Contributions: Consider visibility changes

[MrSerth]

For now, contributions can be started and viewed only when the corresponding base task is visible. Since groups only allow full memberships with writing permissions, this means a task must be public to allow contributions.

Per se, requiring a base task to be public is not a problem at all and the functionality works as expected. However, as soon as the visibility level is changed, the problem begins. In the current implementation, a change in visibility means: Contributors immediately loose access to their contributions. This is especially problematic, if the contribution is still pending.

We should investigate what needs to be done when the visibility level changes. For example, it could make sense to display a warning, deny changes to the visibility settings until all contributions were either accepted or rejected, or automatically decouple pending contributions.

When changing this, check:

• scope :pending_contribution in app/models/task.rb
• example TaskContributionPolicy when the user is logged in when the user is the contribution user when the original task is private in spec/policies/task_contribution_policy_spec.rb

Read more about the advantages and disadvantages here.