Contest observation mode "private" has same effect as "protected"

[tom93]

As far as I can tell by looking at the code, setting the contest observation to "private" actually has the same effect as "protected".

The field was added in commit fb341af with the comment

for observers: public (everyone), protected (groups it is added to even if not competing), private (only if competing)

The logic was added in commit b298d94, and only checks if the field is set to "public"; otherwise the behaviour matches the description of "protected".

I don't understand how "private" as defined in the original comment is useful. The main impact is that it hides the scoreboard at the end of the contest, which doesn't seem useful.

It almost helps with an issue discussed in chat:

users that start later have a slight advantage in terms of information, since they are able to see which problems/subtasks have been solved by more users

However, to solve this issue "private" would need to only show the scoreboard after the user finished competing (as opposed to during), and I don't think it makes sense to use the observation field for that; we should use a separate field as prototyped in #103.

I suggest we either

1. Combine "private" and "protected" into a single option called "private", with the current behaviour of non-public contests; or
2. Fix the code to match the documentation (i.e. if observation set to private, only show the scoreboard to users who are currently competing in that contest).

Related: Filelinks also have public/protected/private visibility, where "protected" means the user can view the file if not competing. See issue #106 for discussion.