Cannot copy/paste on iOS

For a while now I have been unable to copy and paste from GitLab.com pages on my iOS devices.

I think this may be related to the custom GFM copy/paste feature that we added back in 8.17 with !8597 (merged).

/cc @DouweM

Fix

• Fix so that GFM copy/paste is possible on Safari/Chrome on iPhone.
• If the above is not feasible, disable GFM copy/paste, and just ensure regular copy/paste is possible.