textanswer flagging

[LucPrestin]

Summary:
Adds flags to textanswer reviews to indicate the need for an discussion about a text answer with other staff members

Resolves #1118

Changes:

• Add TextAnswer.is_flagged boolean field
• Add buttons to toggle text answer flags
• Add view to list only flagged text answers
• enable flagging in the texanswers quick view
• enable flagging in the textanswers normal view (full, unreviewed and flagged)
• make all flag buttons yellow
• fix flag status display
• update tests to new url
• add safeguard to textanswer toggling

[janno42]

i didn't test the functionality yet, but it looks promising so far.

please note that the evaluations suggested in quick review need to be filtered (#1118 (comment)) and that the issue calls for an overview page for all flagged text answers of a semester.

[richardebeling]

Did we forget to properly review this? If so, I'm sorry. There are quite some merge conflicts now, but they should be solvable.

On a quick glance, most of the code looks good. I would, however, try to abstain from "toggle" semantics with values where your local representation might be outdated. Consider this scenario:

Staff user 1 opens the review page. They are interrupted, so they suspend their computer. 90 Minutes later, they open it again, to continue reviewing.
In the mean time, staff user 2 has already flagged the answer. But, since staff user 1 didn't reload the page, they are still shown "not flagged" as state. Staff user 1 now also wants to flag, but the "toggle" at the server actually unflags.

(What makes it even worse here is that user 2 will have no indication, their screen will show "flagged" now -- but even without that: Just always send a "target" state to make your operations idempotent, and we don't have to worry about this)

[janno42]

Will be reimplemented by @niklasmohrin after #1776 is merged