Warning when queue is congested

[Didayolo]

If congestion in the queue detected (e.g. 10+ submissions in "Submitted"):

• add a warning
• prevent new submissions?

The "Submitted" count depends on the queue of the competition (including for default queue).

Problem: sometimes it's stuck on "Submitted", even after service restart, so it may be considered congested indefinitely.

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Suggestions:

• Similarly to Limit CPU usage on the default queue #1141, we could limit the "daily submissions limit" for competitions using the default queue
• Having more workers would be beneficial now that Codabench is widely adopted
• Should we optimize the queueing / ordering algorithm? Right now it is a simple first-in-first-out, so a single competition can block a whole queue.

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Issue following #1446