Add loading icon and form hiding on error to group-level merge request settings

What does this MR do?

This MR adds a loading icon to the group-level merge request settings. It also hides the form if the loading icon is shown or the loading fails due to an API error.

After some UX discussion, it was also determined that the alerts should be moved to be inside the section rather than being shown at the top of the page.

Screenshots or Screencasts (strongly suggested)

Setup & testing

1. Enable the feature flag echo "Feature.enable(:group_merge_request_approval_settings_feature_flag)" | rails c
2. View the Merge request approvals settings at [GDK_HOST]/groups/[GROUP]/-/edit
3. Check the loading icon is shown on load and the form is hidden
4. Save the form and check the success alert is shown
5. Block the [GDK_HOST]/api/v4/groups/[GROUP_ID]/merge_request_approval_setting network request using the browsers network panel (Chrome, Firefox)
6. Save the form and check the saving alert is shown
7. Refresh the page and check the loading alert is shown and the loading icon continues to be shown rather than the form

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #335982 (closed)