Improve merge request approval settings

This issue is a direct follow up from https://gitlab.com/gitlab-org/gitlab-ee/issues/2452. To make the new always-on approvals more usable, we should improve on the Merge request approval settings. The following UX is taken from aforementioned issues.

Project settings

• The design of the table of added approvers is simplified to just two textbooks where you set who can approve (users and groups) and how many.
• The user/group labels in approvals fields should have the avatars before the names.

Merge request widget and settings

• Remove the approvers UI from the new/edit merge request form.
• There is a "Settings" button in the approval section which allows users to configure the approvals directly.
• On the "Expand to set the approvals" screen, if users enable it and save changes, they will see the "01 - Default" screen (in 'Set approvals in merge request widget' section below).

Configure approvals in MR widget	Expand to set the approvals

• When doing changes in the approval settings, if users don't click "Save changes", changes will not be saved.
• The user/group labels in approvals fields should have the avatars before the names.

01 - Default	02 - Expand to configure	03 - Added approvers	04 - Saved changes

Backend changes

@leipert: todo