Fix layout bug for popup on mobile

[gerardo-navarro]

Further Notes

• You are able to reproduce the bug with the production docker container
• The following pull request might already contain the answer, see Fix popup positioning, add response message to API calls for etherpad-lite-client to work ether/ep_comments_page#208 <= the code changes did not resolve the issue; however we got inspired by this
• We need to tackle the issue an separate repository as this functionality is extracted in a separate etherpad plugin, see Fix wrong positioning of new comment popup on smaller screen sizes ep_comments_page#1

Checklist

• This pull request requires to merge this pull request first, seee Fix wrong positioning of new comment popup on smaller screen sizes ep_comments_page#1
• Swith to the etherpad plugin ep-comments_page from the kitsteam