Fixed file templates not being fully fetched in Web IDE

What does this MR do?

Fixes the file templates not being fully fetched in the Web IDE. Previously we would just fetch the first page & then not fetch anymore. Now we fetch until their are no more pages left to fetch.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51912

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