Open WebIDE in fork when user doesn't have access

What does this MR do?

This is a follow-up to https://gitlab.com/gitlab-org/gitlab-ce/issues/51952#note_188782361

When opening the WebIDE in a project where the user doesn't have push access, use the forked project instead of the upstream project. The forked project may or may not exist already at the time the "WebIDE" button is rendered, and the creation of the fork is currently hard-coded to the user's namespace in BlobHelper#edit_fork_button_tag.

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