Allow tenancies to set default importances

In my institution at least, we have an endemic problem with academics simply....not engaging with the importance system -- they just click through without selecting one at all.

Considering that we use importances to drive purchasing decisions, my team would like to put forward the suggestion that when No Importance is removed, the default importance is the least important one OR that this is selectable on a tenancy level so we don't have to deal with a huge influx of "required" purchases that aren't actually required at that level.