Allow approvers list to be edited when the minimum number of approvers is not respected

We currently only validate a new list of approvers by using the old list of approvers. But this mechanism doesn't make sense if the list of approvers is missing. So we should allow in such case to create a CR with a new list of approvers that would be approved by the provided person. Though we need to ensure it couldn't cause any security problems: maybe we should do that only if the approvers have certain rights.

—
Original report as bug:

Prerequisite: configuration of CR to use 2 approvers minimum

Reproduction steps:

• Edit a standard XWiki page (e.g. Sandbox) that doesn't have any approver set: a warning is displayed explaining that the approvers are missing to the page
• Save as change request: the modal request to set the approvers
• Login with one of the approver user added in the list

Expected result:

• the created CR should use the list of approvers set
• a badge should be displayed explaining the user is approver

Obtained result:

• the CR has an empty approver list
• no badge is put on the diff view specifying that the logged in user is approver