Fix easy days causing load balancer to disproportionately schedule graduates to the furthest day

[jakeprobst]

In the case where easy days is effectively disabled (all days are set to the same ease), do not call its corresponding routines.

Take this case of graduating intervals:

Here are the intervals with only the load balancer operating
(format is interval: cards_due_on_day, normalized_weight (raw_weight)):

   10: 86 0.15 (0.000013520822)
   11: 81 0.15 (0.000013855983)
   12: 76 0.16 (0.000014427517)
   13: 73 0.16 (0.000014434805)
   14: 73 0.15 (0.000013403748)
*  15: 58 0.22 (0.000019817679)

However after the easy days logic gets run (but all days are all set to Normal), the intervals become

   10: 86 0.00 (0)
   11: 81 0.00 (0)
   12: 76 0.00 (0)
   13: 73 0.06 (0.000021652208)
   14: 73 0.05 (0.000020105623)
*  15: 58 0.89 (0.0003269917)

We're effectively back to the old problem of graduates getting disproportionately placed on the furthest day due to the inherent dip that is created at that point.

Interestingly, having at least one day changed does provide a reasonable-enough load balance:
monday minimal:

   10: 86 0.04 (0.000045970817)
   11: 81 0.10 (0.00011639028)
   12: 76 0.16 (0.00019332876)
   13: 73 0.19 (0.00023673082)
   14: 73 0.00 (0)
*  15: 58 0.51 (0.0006222751)

tuesday minimal:

   10: 86 0.06 (0.000045970817)
   11: 81 0.14 (0.00011639028)
   12: 76 0.24 (0.00019332876)
   13: 73 0.29 (0.00023673082)
*  14: 73 0.27 (0.0002198215)
   15: 58 0.00 (0)

[L-M-Sherlock]

LGTM

[dae]

Thank you both!