Increase runner reuse and allow learners to request a new one

[MrSerth]

As a administrator,
I want to increase the number of runners being reused
in order to improve overall performance in the Nomad cluster.

As a learner,
I want to request a new runner in case my runner is not working as expected
in order to continue learning at any time and don't wait for the runner to expire.

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Things to do:

• Check code occurrences where we destroy_at_management
• Don't delete a runner if code execution is stopped by the learner
• Add a small link to the /implement view to allow learners to request a new runner
• Consider a rate limiting for requesting a new runner