Clean up code in preparation for role details drawer refactor

What does this MR do and why?

On the Manage -> Members page, there's a list of members. Clicking on the role for any member will show the role details drawer:

This MR is a prerequisite for !157008 (merged), to reduce the size by breaking some of it out to a separate MR. This MR does the following:

1. In members_table.vue, move memberPath and namespace directly into each member object to make them significantly easier to consume.

2. In guest_overage_confirmation.vue, rename confirmOverage to checkOverage.

3. In members/utils.js, add several shared functions for getting and setting the member role.

How to set up and validate locally

1. Enable the show_role_details_in_drawer feature flag:

echo "Feature.enable(:show_role_details_in_drawer)" | rails c

2. Go to any group/project -> Manage -> Members.

3. Click on the role for a member that's a direct member.

4. Verify that the drawer shows.

5. Change the role and click on Update role. Verify that the role is updated.

Related to #464104 (closed)