Swap backlog label with a project custom field

[robrap]

A.C.

• Add "Backlog" custom field.
• Mark all our items in the backlog view with the new backlog custom field (and display the new custom field on this view)
• Update the backlog view to display the new field, and other views that filter out backlog should change how they filter.
• Mark Expand dependabot alert count checks to all repos openedx/edx-repo-health#366 with the new Backlog custom field.
• Move Restore coverage checks of xqueue openedx/xqueue#887 to the backlog once we have this new field.

Thoughts:

• Maybe the Backlog field is a list of options, and the only option is also named "Backlog" (or "In Backlog"), and for non-backlog items we leave it blank so it doesn't show anything on the board.

[robrap]

Thanks for doing this @dianakhuang.

1. I updated one of the views that still was referring to the label.
2. Since I removed the label from a ticket and expected it to go to the board, but it didn't, I realized that it was going to be confusing to keep the old labels around. So, I removed them all and now we're just using your new custom field.
   Thanks again! Looks great. Also, I'm going to close this issue.