Add ability to sort backlog by labels

Problem to solve

It's currently too much manual work to sort backlog

Further details

If labels are assigned a numerical value then the combined value of all attached labels should work to automatically sort the backlog

a zero value should be possible on a label so not all increment the value

Negative values should also be possible.

Proposal

Add ability to weight labels and sort backlog by this weighting

What does success look like, and how can we measure that?

I no longer have to manually sort the backlog

Links / references

ZD: https://gitlab.zendesk.com/agent/tickets/102898 (Internal Use Only)