Add accidentally deleted project config for custom apply suggestions

What does this MR do?

I've created !21411 (merged) which was merged into 12.7 and announced in a release post (see https://about.gitlab.com/releases/2020/01/22/gitlab-12-7-released/#configure-default-commit-message-for-applied-suggestions). It seems that an unrelated commit removed parts of this MR, so the configuration in the UI won't work, see: 66c9cbef

This MR intends to fix this in a timely manner. I'll provide some further specs to ensure project level configuration as a follow-up MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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