Remove private TextAnswers

[janno42]

Private TextAnswers were added (in #530) for this purpose: A tutor/PhD student receives valuable feedback that we want to give them but don't want the responsible professor to see (e.g., because it directly or indirectly criticizes the professor). Back then, responsibles could see all TextAnswers.

Since #1279, responsibles can no longer see all TextAnswers, only those about themselves and those for general questions. The only case in which users can see other users' TextAnswers (besides reviewers/managers) is if they are delegates of those users. Usually, PhD students can be delegates of professors, but hardly the other way around. So the original use case no longer exists.
Private TextAnswers have rarely been used in recent semesters (and those instances aren't critical), so we can

• remove private TextAnswers completely from the codebase.
• All TextAnwers currently marked as "private" should be changed to "public".
• The ReviewDecisions should be renamed to "publish"/"delete"/"undecided".

[niklasmohrin]

Just a thought while we are at it, how do we feel about something like "keep" instead of "publish"? Or maybe "approve"? There isn't really a reason to change, but we might as well pick the one we like best while we rename it

(The discussion from the previous rename is #1776)