discount priority to use sortable

[wychoong]

Is your feature request related to a problem? Please describe.
Currently Discount only sort by Low, Medium, High and is quite limited

Describe the solution you'd like
since the priority column for Discount is already integer and storing 1: Low, 5: Medium, 10: High
the required changes I foresee:

1. remove priority section in discount create/edit
2. implement a list with sortable functionality similar to Collection page to replace current list page table

Describe alternatives you've considered
to have sortable functionality directly on lunar-table