Resolve "WebIDE doesn't work on empty repositories"

What does this MR do?

Resolves:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/45687 by supporting empty-state view for empty or non-existent branches in Web IDE, and
• https://gitlab.com/gitlab-org/gitlab-ce/issues/60107 by updating the heading in Web IDE's empty state

UPDATE The MR also includes handling of the Web IDE's state after first commit to non-existing branch

NOTE There is an EE port of this branch available

What are the relevant issue numbers?

Closes #45687 (closed) Closes #60107 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer