Create merge request approval settings vue app with store

Why are we doing this work

To facilitate the addition of group-level merge request approval settings; we need to initially set up a containing Vue app and store to control the settings and their submission in one localised place.

This lays the groundwork for adding each setting in future.

Non-functional requirements

• Feature flag: group_merge_request_approval_settings_feature_flag

Implementation plan

• If the feature flag is enabled Add new Merge request approvals section to the Groups general settings page (title, text and link the same as projects)
• Create new Vue app in ee/app/assets/javascripts/approvals to render the contents of the section.
  • Note: This should be written in a way to support both groups and projects in future as one ultimate goal would be to unify the project and group merge request approval settings for ease of maintenance and future development
• Create new store in ee/app/assets/javascripts/approvals to handle the API communication for retrieving, locking and saving the MR approval settings
• Add Save changes button which makes an API request to send the data
• Make sure failures are noted in a banner
• Make sure successfully saving is noted in a banner
• Add tests for the new app and store

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