Rename and add keys/values in `/api/status/crawler` response for more clarity

[bolinocroustibat]

Our /api/status/crawler keys naming is a bit poor.

Proposal:

For the /api/status/crawler route, instead of having as currently:

"checks": {
    "pending_count": count_pending_checks,
    "fresh_count": count_fresh_checks,
    "checked_percentage": rate_checked,
    "fresh_percentage": rate_checked_fresh,
},

we could have:

{
  "checks": {
    "in_progress_count": ...,
    "in_progress_percentage": ...,
    "needs_check_count": ...,
    "needs_check_percentage": ...,
    "up_to_date_check_count": ...,
    "up_to_date_check_percentage": ...,
  }
}

I don't think it requires additional SQL queries, these are complementary numbers.

[bolinocroustibat]

Done in #387 with other refactor