Add "Custom HTTP clone host" setting

What does this MR do?

Allows customization of project HTTP clone URLs.

Screenshots

Added to Admin Area > Settings > General > Visibility and access controls below Enabled Git access protocols:

When set to mike.example.com, the clone panel for a project looks like:

Though you can see a problem there-- the port is unchanged from 3001. What if the sysadmin wants to change the port? Or the schema from HTTP to HTTPS or vice-versa?

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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