Add separate form input for user when push or pull mirroring

What does this MR do and why?

Passes Username directly to ProjectImportData without using username_only_import_url or import_url.

Creates a form input for username when push or pull mirroring.
This removes the requirement/ability to add username via the repo url.
This allows us to remove the virtual accessor username_only_import_url
and simplify the accessor import_url, which have been a rich source of bugs.

Contributes to Take username and password separately for proje... (#19382 - closed)

Screenshots or screen recordings

New field below. Can be found in project settings / repository / mirroring repositories.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Example below:

1. Visit any project repository settings and expand 'Mirroring repositories' such as http://127.0.0.1:3000/jerry.waters/lab-coat/-/settings/repository
2. Add new mirror repository.
3. Do not include username in 'Git repository URL' as one would previously.
4. Select Authentication method 'Username and Password'
5. Add a new mirror with username and password.
6. A new mirror should be added to the project mirrors below.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #19382 (closed)