Only make the Web IDE the default editor for desktop browsers

Problem to solve

The Web IDE does not have good support for mobile devices because it is built on Monaco which does not support mobile devices. The Ace editor used by the single file editor doesn't have great mobile support, but it is better.

The known issues relating to iPad problems are documented in &756 (closed)

Proposal

Modify the feature flag added by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25171 so that it only makes the Web IDE the default editor for desktop devices.

Links / references

&756 (closed)