Add option to recalculate object's Importance individually

[mikron-ia]

Currently, the only way to recalculate object's Importance is to recalculate everything. #401 may fix that to a degree but introduction of #400 will make single-target recalculations feasible.

Implement a command to manually recalculate a given object's Importance, with object's id (or ImportancePack id, whatever makes more sense in implementation) provided as parameter. This will be later callable by queue-fed consumer.

The ability to recalculate everything flagged and everything without concern for a flag should be retained for the purpose of handling potential queue failures or bugs.