Deprecate GET /admin/application_settings route

What does this MR do?

Makes Admin Area > Settings > General page accessible primarily at /admin/application_settings/general path, and deprecates /admin/application_settings. A temporary redirect from the former to the latter path is introduced, this redirect is to be removed in GitLab 13.0: #199427 (closed).

The motivation for this MR is described in #43659 (closed).

Closes #43657 (closed), #43658 (closed) and #43659 (closed)

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.

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