Warning when queue is congested

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

• add a warning
• add a confirmation pop-up when trying to make a new submission

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.

Suggestions:

• Similarly to #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.

Issue following #1446

The issue names no files, tests, or entry points. Start by locating the competition queue and the handling of submissions in “Submitted,” then verify how queue congestion and stuck submissions are represented. Done means a congested queue is detected, users receive a warning and confirmation before submitting, and the behavior is covered by appropriate tests.