Conditionally display link text on welcome page

What does this MR do?

Displays different link text on the onboarding welcome page depending on where the user gets redirected from (see also the following comment)

• Auto-redirect for new users: No, not interested right now
• "Learn GitLab" from help menu: Skip this for now

Screenshots:

Auto-redirect for new users	"Learn GitLab" from help menu

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