Add ability to turn off merge request approvals feature entirely

Problem to solve

As a user of the Core Omnibus edition of GitLab, I want to disable the merge request approval feature introduced to core in 13.2.

The feature adds unnecessary UI bloat to the MR interface, and without the ability to define any rules in Core, it is completely superfluous to our needs.

Typically these kinds of features can be disabled at the instance or project level, but after searching for such an ability I couldn't find it.

The value added by making removing this feature configurable is streamlining the review screen when the approval workflow is not needed.

Intended users

Unknown as the goal is to not use the approval feature.

Configuration applied by administrators, simpler UI enjoyed by developers.

User experience goal

Simpler UI & merge request workflow when approvals are not needed.

Proposal

Enable a configuration option either in gitlab.rb for the instance, or a setting in the admin settings to disable MR approval entirely.

Alternatively making this a project-level feature would be acceptable, but tedious.

Further details

N/A

Permissions and Security

N/A

Documentation

Documentation for instance configuration options would need updated.

Feature documentation for MR approvals could be updated to mention the feature can be disabled by your admin.

Availability & Testing

What does success look like, and how can we measure that?

Ability to turn off MR approvals entirely
Once the feature is disabled, the MR UI should no longer show any approval-related elements