Use ee_else_ce alias to resolve merge request widget files (EE-port)

EE-port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24374

What does this MR do?

Introduce new ee_else_ce alias in Webpack to resolve files which are different in CE and EE.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/53181

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?