Set low priority for sandbox projects in the backend

[ramyaragupathy]

There is no use providing an urgency/priority indicator for sandbox projects because they are not related to activation projects producing live data. Hence we propose to remove the need for priority for these projects. However removing the need for priority entirely would be a significant heavy lifting in the backend considering the importance of this metadata field for the live projects.

So @prabinoid has propsoed to offer a default low priority option for sandbox projects. Frontend changes to be implemented separately. cc @suzit-10

[linear]

TECH-973