Edit Branch Rules: Approval Rules: Update frontend app to use Drawer component instead of Modal

Context

See [MVC] Branch Rules Editing (&8075 - closed)

Tasks

For the approval rules to be editable inside Branch Rules, we'll need to:

• Move the Add/Edit modals to Drawers
  • Use the User selector
  • Use the Group selector

Other considerations:

• Minimum required approval
  • Same as "All eligible users" in current merge request approval but now there is a name for the rule
  • Default is 0.
• When there are no merge request approvals setup, display "Merge request approvals - Default"
• Add approval rule dropdown button with two options
  • "Add approval rule"
  • "Add coverage check"
• Editing done in drawer
• Upon update of approval rule, display toast "Approval rule updated"
• Upon removal of approval rule, display toast "Approval rule removed"
• Same confirmation modal as current production should be displayed when click "Delete" from list view

Open questions

• Can we apply the Drawer to all usages of Approval Rules app?
  • Implementation note: If not, we risk incurring in a drastic increase of tech debt in this frontend app.