Refactor Scheduler parameters

[HebaruSan]

Motivation

This is a quick follow-up to #348, after which I was confused by this "200" because the default for the min_cpu parameter is 50:

Cause

Defaults for these parameters were being specified in multiple places, with different values, and only one of them was being used.

Changes

• Now the limit parameters to can_schedule no longer have defaults, so it is much more clear that the actual defaults come from the @click.option attributes on scheduler
  • To do that, the dev parameter is now last so it can retain its default
• Now --min-gh can be passed on the command line to specify the GitHub API rate limit threshold, with the param's previous default of 1500
• Now --max-queued's help string describes that parameter instead of some other parameter