Add popover to approval settings help icons

Problem to solve

As noted by @eread in !66536 (comment 644644788):

You could update the ? icons to text links, per: https://design.gitlab.com/usability/helping-users#formatting-help-content.

Proposal

Add a GlPopover to each settings' ? icon in ApprovalSettings with a title and short description.

Setting	Title	Description
Prevent MR approvals by the author.	What effect will this have?	When enabled, the author of a merge request cannot approve it.
Prevent approval of merge requests by merge request committers.	What effect will this have?	When enabled, users who have committed to a merge request won't be able to approve it.
Prevent users from modifying MR approval rules.	What effect will this have?	When enabled, users won't be able to override the project's approval rules on merge requests.
Require user password for approvals.	What effect will this have?	Force potential approvers to first authenticate with a password.
Remove all approvals in a merge request when new commits are pushed to its source branch.	What effect will this have?	When enabled, remove all existing approvals on a merge request when more changes are added to it.