Rename "Benchmark" into "Task" (for disambiguation)

[d-uzlov]

Currently we use the word "benchmark" for both individual benchmarks of one model, and for the benchmark suite, or the app as a whole.
It would be easier to understand if we rename individual benchmarks to "tasks".
We already use this name in some places.

[anhappdev]

We can either rename benchmark -> task or task -> benchmark since the 2 terms are used interchangeable here.
Actually, renaming task -> benchmark would be easier since it does not impact the backend code (all the backend settings currently use the term benchmark_setting and benchmark_id)