Treatment of error message for a lab with unassigned graders

[Evgeny Kotelnikov]

Currently, if a lab doesn't have any graders assigned, an attempt to go on a lab page fails with an ugly nginx message. Of course, graders are supposed to be assigned before hand, but a proper treatment of their absense in still necessary. I see two solutions:

1) Show proper error message, saying that submissions to a lab are temporarily on hold. 2) Disable the link to the lab on the front page altogether with similar a message.

I like the second one more.

[Evgeny Kotelnikov]

Don't show a link to a misconfigured lab (fix #36)

In particular, to the ones without graders assigned. Instead, say that submissions to the lab are "temporarily suspended".

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