Draft: Add company name and employee count to sign-up welcome page

What does this MR do?

It adds company/organization name and number of employees fields to the sign-up welcome page, where the fields would only be shown when the user selects the "setup for company" option.

The MR adds the two fields to the UserPreference model and uses the fields on the frontend.

Related to gitlab-org/customers-gitlab-com#2387

How to produce

• Create a new account in local GitLab
• After clicking the Register button, you'll be redirected to the welcome page (where these changes will take effect)

Screenshots

Just me is selected (no changes)	My company or team is selected (show 2 new input fields)

Total number of employees is required	All fields are populated

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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.